Abstract: The effect of off-axis use of Cassegrain telescope with different objectives on heterodyne detection probability is studied. In the non-cooperative target heterodyne detection system, when light is off-axially emitted from Cassegrain telescope, the emitted energy is increased while detection probability is declined. In cooperation target heterodyne detection system, the emitted energy is increased by 30%, detection probability is also increased greatly. The detection probabilities for the scattering object and mirror measured at laboratory are 17% and 85% respectively. It verifies that off-axis Cassegrain telescope can be used for achieving better performance in cooperation target heterodyne detection system.