Discrete signal is one of the most essential signals in ground simulation test for civil aircraft. Based on the test requirements of some civil aircraft, this paper put forward a simple and high reliable discrete signal output circuit design . The design transformed TTL signal into discrete signal by using CMOS and photoelectric coupler as isolation. The circuit simulation software Multisim was adopted to analyze the function of discrete signal output circuit. Experimental verification was done in iron bird of some civil aircraft. The results of simulation and test show that the discrete signal output circuit meets the practical requirements.