In this paper, a new transponder design method suitable for multi-satellites parallel testing is proposed to improve the test efficiency and to reduce the test cost. The bottleneck problem of multi-satellites parallel test is solved by the ground testing equipment and transponder using the same interface of tele-command RS422 and the interface of telemetry RS422. It has been successfully applied to the parallel testing of a constellation, which reduces the testing cost, improves the testing efficiency and shortens the testing cycle of the constellation.