Abstract: Monte Carlo program was used to simulate the output spectrum of a welltype antiCompton spectrometer with the main probe wrapped with NaI(Tl) crystal. We gave the antiCompton suppression spectrum and Compton electron spectrum by adjusting the parameters of the FU card to screen the pulse height of the external detector. The usage method of FU card
 of MCNP in anticoincidence measurement was verified, and the effect of the thickness of external detector on antiCompton spectrum was analyzed.

Key words: Monte Carlo simulation, Compton suppression, thickness of external detector