Abstract: It is difficult to work out the analytical solution when solving the extension and evaporation model of LNG leaking to water surface. Given constant leak rate, the perturbation method is used to non-dimensionalize the parameters in the formulas of the model, so as to solve the approximate analytical solution of liquid bath's mass, radius, evaporation rate and thickness. Compared with the conventional numerical simulation method, the application of perturbation method can simplify the mathematical problem and solving process to obtain clear analytical structure. It can avoid the problems existing in the numerical solution of computers including the accuracy, stability and astringency of solutions. Considering the specific leaking cases, the solved high order perturbed solution is verified with the numerical solution obtained with the Runge-Kutta method. The results show that the high order perturbed solution fits the numerical solution very well.