This paper describes a new wet etching simulation that considers physical phenomenon such as flow, diffusion and chemical reaction. A partial differential equation with implicit parameterization of etching properties is presented, which is solvable by commercial software. Three implicit parameters associated with flow, diffusion and chemical reaction are determined through experiment by test device so that the simulation fits the experimental result. For automatic parameter identification, an error function was proposed considering position of etching front at the final target time, average of etching rate and shape of etching front. The central composite design algorithm was adopted with quadratic interpolation and minimum least square approximation to reduce the computational cost. Once the parameters are determined, the etching simulation of real MEMS device is practically carried out.