In order to resolve the problem that the firing accuracy of shipborne gun descended due to the variance between ship actual position and calculative position, improve the measurement method. Use ship position longitude and latitude coordinate, course and radar to measure water column distance and relative bearing, calculate the water column longitude and latitude coordinate. Taking the shipborne gun firing point as the coordinate origin, calculate the derivation of water column predicted point distance and direction. The simulation results show that firing accuracy of the improved method was more accurate and influenced rarely by sea condition, and the accuracy superiority was obvious when the firing distance was far.