To test the vehicle electronic control units, an improved test case generation algorithm based on the IPO (In-Parameter-Order) strategy is proposed via the parameter correlation and combination constraints of the vehicle. An integration testing system for vehicle electronic control units is developed, in which a unified management and data sharing mechanism is enabled for the common equipments used in vehicle test. The proposed method is validated under two different kinds of vehicle electronic control units and test conditions. The results show that the proposed test case generation algorithm can achieve the same coverage with a clear reduced number of test cases. And the integrated test system can meet the test requirements of the different vehicle electronic control units, with an improved testing efficiency and accuracy.