The LiYF4single crystals singly doped Ho3tand co-doped Ho3t,Pr3tions were grown by a modified Bridgman method.The JuddeOfelt strength parameters(U2,U4,U6)of Ho3twere calculated according to the absorption spectra and the JuddeOfelt theory,by which the radiative transition probabilities(A),fluorescence branching ratios(b)and radiative lifetime(srad)were obtained.The radiative lifetimes of5I6and5I7levels in Ho3t(1 mol%):LiYF4are 10.89 and 20.19 ms,respectively,while 9.77 and 18.50 ms in Ho3t/Pr3tdoped crystals.Hence,the sradof5I7level decreases significantly by introduction of Pr3tinto Ho3t:LiYF4crystal which is beneficial to the emission of 2.9 mm.The maximum emission cross section of Ho3t:LiYF4crystal located at2.05 mm calculated by McCumber theory is 0.51 10 20cm2which is compared with other crystals.The maximum emission cross section at 2948 nm in Ho3t/Pr3tco-doped LiYF4crystal obtained by Fuchtbauere Ladenburg theory is 0.68 10 20cm2,and is larger than the value of 0.53 10 20cm2in Ho3tsingly doped LiYF4crystal.Based on the absorption and emission cross section spectra,the gain cross section spectra were calculated.In the Ho3tions singly doped LiYF4crystal,the gain cross sections for 2.05 mm infrared emission becomes positive once the population inversion level reaches 30%.It means that the pump threshold for obtaining 2.05 mm laser is probably lower which is an advantage for Ho3t-doped LiYF42.05 mm infrared lasers.The calculated gain cross section for 2.9 mm mid-infrared emission does not become positive until the population inversion level reaches 40%in Ho3t/Pr3t:LiYF4crystal,but 50%in Ho3tsingly doped LiYF4crystal,indicating that a low pumping threshold is achieved for the Ho3t:5I6/5I7laser operation with the introduction of Pr3tions.It was also demonstrated that Pr3tion can deplete rapidly the lower laser Ho3t:5I7level and has influence on the Ho3t:5I6level.The Ho3t/Pr3t:LiYF4crystal may be a potential media for 2.9 mm mid-infrared laser. The LiYF4single crystals singly doped Ho3tand co-doped Ho3t, Pr3tions were grown by a modified Bridgman method. The JuddeOfelt strength parameters (U2, U4, U6) of Ho3twere calculated according to the absorption spectra and the JuddeOfelt theory, by which the radiative transition probabilities (A), fluorescence branching ratios (b) and radiative lifetime (srad) were obtained. The radiative lifetimes of 5I6 and 5I7levels in Ho3t (1 mol%): LiYF4are 10.89 and 20.19 ms, respectively, while 9.77 and 18.50 ms in Ho3t / Pr3tdoped crystals. Therefore, the sradof5I7level decreases significantly by introduction of Pr3tinto Ho3t: LiYF4crystal which is beneficial to the emission of 2.9mm. The maximum emission cross section of Ho3t: LiYF4crystal located at 2.05mm calculated by McCumber theory is 0.51 1020cm2which is compared with other crystals The maximum emission cross section at 2948 nm in Ho3t / Pr3tco-doped LiYF4 crystal obtained by Fuchtbauere Ladenburg theory is 0.68 10 20 cm 2, and is larger than the value of 0.53 10 20 cm 2 in H o3tsingly doped LiYF4 crystal. Based on absorption and emission cross section spectra of the gain cross section spectra were calculated. In Ho3tions singly doped LiYF4crystal, the gain cross sections for 2.05 mm infrared emission becomes positive once the population inversion level reaches 30% .It means that the pump threshold for obtaining 2.05 mm laser is probably lower which is an advantage for Ho3t-doped LiYF42.05 mm infrared lasers. calculated gain cross section for 2.9 mm mid-infrared emission does not become positive until the population inversion level reaches 40% in Ho3t / Pr3t: LiYF4crystal, but 50% in Ho3tsingly doped LiYF4crystal, indicating that a low pumping threshold was achieved for the Ho3t: 5I6 / 5I7laser operation with the introduction of Pr3tions. It was also demonstrated that Pr3tion can deplete rapidly the lower laser Ho3t: 5I7level and has influence on the Ho3t: 5I6level.The Ho3t / Pr3t: LiYF4crystal may be a potential media for 2.9mm mid-infrared laser.