An instrument to detect atmospheric HO2 radicals using fluorescence assay by gas expan-sion(FAGE)technique has been developed.HO2 is measured by reaction with NO to form OH and subsequent detection of OH by laser-induced fluorescence at low pressure.The sys-tem performance has been improved by optimizing the expansion distance and pressure,the influence factors of HO2 conversion efficiency are also studied.The interferences of RO2 radicals were investigated by determining the conversion efficiency of RO2 to OH during the measurement of HO2.The dependence of the conversion of HO2 on NO concentration was investigated,and low HO2 conversion efficiency was selected to realize the ambient HO2 measurement,where the conversion efficiency of RO2 derived by propane,ethene,isoprene and methanol to OH has been reduced to less than 6％in the atmosphere.Furthermore,no significant interferences from PM2.5 and NO were found in the ambient HO2 measurement.The detection limits for HO2(S/N = 2)are estimated to 4.8 × 105 cm-3 and 1.1 × 106 cm-3(pHO2 = 20％)under night and noon conditions,with 60 sec signal integration time.The instru-ment was successfully deployed during STORM-2018 field campaign at Shenzhen graduate school of Peking University.The concentration of atmospheric HOx radical and the good cor-relation of OH with j(O1D)was obtained here.The diurnal variation of HOx concentration shows that the OH maximum concentration of those days is about 5.3 × 106 cm-3 appear-ing around 12:00,while the HO2 maximum concentration is about 4.2 x 108 cm-3 appearing around 13:30.