As the key part of vehicle control, ECU must be tested after manufacturing. This online function test system aims to automatically test the hardware and software of ECU PCB after they are welded. A test system consisting of upper PC, test board, and tested ECU is built. According to the feature of the ECU (mutibus based, hardware and software included), the communication protocol, as well as testing software for upper and lower devices is developed. This system is easy to be applied and expanded in related ECU PCB testing. Batch test with this test system for newly-produced ECU is implemented, which ensures high quality and stability of the products.