In the integrated navigation system, which consists of a laser Doppler velocimeter (LDV) and strapdown inertial navigation system, it is necessary to calibrate the LDV. We propose an online calibration method for a LDV based on the strapdown inertial navigation system. This system uses highly accurate output information in the early stage of the inertial navigation system to carry out coarse calibration for the velocimeter and then employs a Kalman filter to correct the results of coarse calibration to achieve high precision calibration for the velocimeter. Experimental results show that the relative distance error of dead reckoning is less than 0.02%, indicating that the proposed online calibration method can achieve high-precision calibration for the velocimeter.