Based on the microcontroller C8051F340, a roadway test system was designed for motor vehicle's longitudinal performance testing, such as acceleration and braking performances. The sensing device was comprised of GPS system, braking switcher, steering angle sensor and combined sensor, etc. The last two sensors were selected from vehicle electronic stability control system ( ESP ), which offer CAN (Controller Area Network) interface and all the data update frequency was 100 Hz. The GPS device's data update frequency is only 1 Hz, but its accuracy of speed measurement is 0.1 km/h. In order to get the precise travel distance and 100 Hz speed information, a data fusion algorithm was employed based on the speed information from GPS and the vehicle longitudinal acceleration information from the combined sensor. These sensors were selected according to the requirements of the laws and regulations of vehicle performance test. All the sensors' interface protocol, system hardware design, microcontroller and portable computer's program design were introduced in detail. Finally, a confirmatory road test was conducted to verify the system’s correctness and reliability. With those advantages of low cost, expandable interface (CAN bus) and moderate accuracy, the designed system can offer an advanced test platform for the research of vehicle braking and acceleration performances.