A high-overtone bulk acoustic resonator(HEAR) is composed of a substrate,a piezoelectric film and upper and lower electrodes,the influences of their structure parameter(thickness) and performance parameter(characteristic impedance) on effective electromechanical coupling coefficient K_(eff)~2 are investigated systematically.The relationship between K_(eff)~2 and these parameters is obtained by a lumped parameter equivalent circuit instead of distributed parameter equivalent circuit near the resonant frequency,and K_(eff)~2 at the resonance frequency closest to the given frequency is analyzed.The results show that K_(eff)~2 declines rapidly and oscillatorily with the continuous increase of the substrate thickness when the piezoelectric film thickness is fixed,and decreases inversely proportion to the thickness when the substrate thickness is greater than a certain value.With the ratio of the characteristic impedance of the substrate to the piezoelectric layer increasing,the maximum of K_(eff)~2 obtained from the variation curve of K_(eff)~2 with the continuous increase of the piezoelectric film thickness decreases rapidly before reaching the minimum value,and later increases slowly.Fused silica with low impedance is appropriate as the substrate of HBAR to get a larger K_(eff)~2.Compared with Al electrode,Au electrode can obtain larger K_(eff)~2 when the appropriate electrode thickness is selected.The revealed laws above mentioned provide the theoretical basis for optimizing parameters of HBAR. A high-overtone bulk acoustic resonator (HEAR) is composed of a substrate, a piezoelectric film and upper and lower electrodes, the influences of their structure parameter (thickness) and performance parameter (characteristic impedance) on an effective electromechanical coupling coefficient K eff ~ 2 are characterized systematically. The relationship between K eff ~ 2 and these parameters is obtained by a lumped parameter equivalent circuit instead of distributed parameter equivalent circuit near the resonant frequency, and K eff ~ 2 at the resonance frequency closest to the given frequency is analyzed. The results show that K_ (eff) ~ 2 declines rapidly and oscillatorily with the continuous increase of the substrate thickness when the piezoelectric film thickness is fixed, and inverse inversely proportion to the thickness when the substrate thickness is greater than a certain value. With the ratio of the characteristic impedance of the substrate to the piezoelectric layer increasing, the maximum of K _ (eff) ~ 2 obtained from the variation curve of K eff ~ 2 with the continuous increase of the piezoelectric film thickness decreases rapidly reaches reaching the minimum value, and later increases slowly. Fused silica with low impedance is appropriate as the substrate of HBAR to get a larger K ~ (eff) ~ 2.Compared with Al electrode, Au electrode can obtain larger K ~ (eff) ~ 2 when the appropriate electrode thickness is selected. The illustrated laws above mentioned provide the theoretical basis for optimizing parameters of HBAR.