Abstract: The authors introduce the basic principle of laser range finding technology based on phase, propound in detail the automatic digital measurement technique of phase difference and its errors, analyze the effect of single phase-picking precision, frequency drift and atmosphere refractive index, etc. on laser ranging errors and put forward some special improvement methods. The results of laser ranging realization show that adopting phase laser ranging can achieve the precision of ±(5 mm 4+5×10^-6D).