To depend on the development platform of the smart car contest, a smart car servosystem, based on Freescale 16-bit HCS12 micro-controller MC9S12XS128, is designed in this paper. The system uses the low-power CMOS digital camera OV6620 to obtain the real-time track information, and extracts the black line on the contest lane with the edge detection method to calculate the position deviation between the car and the black line, with the purpose of distinguishing the different shape of the lane. The nonlinear PD control algorithm for the steering control was given in this paper. Through the photo electrical encoder named YZ30D4S-2NA-200 to attain the real-time car speed, the system uses the nonlinear P strategy to achieve the speed closed-loop control. It is proved that the servosystem can achieve the automatic line patrol control, so as to make the smart car drive stably and quickly.