The combined speed log is one of the shipborne key navigation equipment. The accuracy of velocity measurement of the combined speed log can normally meet the requirements only if it is calibrated by using the method of multiple targets chasing on speed testing site. Calibrating annually the combined speed log on speed testing site costs in huge human power and time. In addition, the accuracy of velocity measurement of it is under the influence of weather, current and sea states. How to calibrate effectively the combined speed log by the use of shipborne equipment is discussed. The researches on the velocity measurement method, the range calculation equations and the analysis of spending time in velocity measurement are made to draw the conclusion that the application of GPS receiver to the calibration of the combined speed log is perfectly feasible.