A high accuracy of estimation of speed-lose in wave is an important problem demanding prompt solution in navigating practice. In this paper, the estimated equation educed by observations at sea is considered as an optimization method to solve the problem, so Aertssen formula can be used as the basis of the optimization method, which needs to be modified and improved. The modified formula given in this paper can be used in the estimation of speed loss in wave and the designment of ship's seakeeping in lack of navigation data. The improved formula is also presented, which compatible with real marine environment, and it can improve the precision of speed loss estimation in wave. The equation provided is validated.