In photorefractive two-wave mixing system with incoherent background beam, many parameters in system will affect the efficiency of mixing. To solve the optimal designing of parameters, a new method based on stochastic resonance (SR) was put forward. Firstly, by analyzing the two-wave mixing process through physical and numerical way, the stochastic resonance essence was discovered in the two-wave mixing. Secondly, a new method for optimal matching parameters designing was suggested from the aspect of SR. In this new method, for the given gain requirement, the length of crystal was decided according to the saturation of signal firstly, and then the ratio of incoherent light was decided by its affection to gain. Numerical results showed that both the SNR and the gain are improved after the optimal designing of parameters.