Abstract: In the designed auto adaptive cruise control system,infrared laser ranging measures the distance between the main and the objective vehicles.The wavelength of the laser and the precision of the distance are very important to the reliability of the system.The factors affecting the accuracy of laser ranging are analyzed.Then the digital interpolating time counting method has been proposed to produce the high precision of distance measuring.Considering factors such as attenuation,reflectivity,system cost and eye safety,a 1.6 μm infrared laser wavelength is suggested as optimum.And the laser driving circuits and the detect-amplifying circuits are introduced.