Two schemes such as single pixel signal to noise ratio and extinction ratio are used to evaluate transmit-receive isolation ability based on the practical application of the tracking and communication detector in laser communication system. A method to inhibit the back scattering light was presented which boring a hole in the secondary mirror and increasing light trap. It is derived that when aperture ratio of secondary mirror K is 0.1~0.3, it dose not affect the emission efficiency,the measured results show that the improved Cassegrain optical antenna Transmit-Receive isolation is lower than －40 dB while not affecting the emission efficiency of system, and meet the requirements of laser communication system acquiring, tracking and communication.