SMS4 is the first official released commercial cryptographic algorithm.It provides unified standards for designing and using local area wireless network product.The general DPA attack is not suitable for SMS4 owing to the ample random diffusion of the round output.This article proposed a new power analysis method for SMS4 to reduce the diffusion by chosen plaintext.Two means-Hamming distance model and bit model-are used to build the power model.Simulation results show that this method is effective and can be used in actual cryptographic circuit such as smart cards.