Abstract: In order to detect gamma rays with different energy at the external target experimental terminal of the Cooling Storage Ring (CSR) of Heavy Ion Research Facility of Lanzhou, a gamma ray detector with perfect energy resolution is being constructed at Institute of Modern Physics, Chinese Academy of Sciences. It is made of CsI(Tl) crystal grown at IMP. The light output of non-uniformity and energy resolution of CsI(Tl) crystal were tested after CsI(Tl) crystal coupled with the Hamamastu S8664-1010 avalanche photodiode (APD). From the result of test, standard of CsI(Tl) crystal qualified was obtained. At present, one-sixth of the detectors was finished. The test result indicates that 1 024 pieces are good enough, and the percent of pass is over 94%.