Focused on the low analysis efficiency of Correlation Power Analysis (CPA) interfered by noise, a stepwise CPA scheme was proposed. Firstly, the utilization of information in CPA was improved by constructing a new stepwise scheme. Secondly, the problem that the accuracies of previous analyses were not guaranteed was solved by introducing the confidence index to improve the accuracy of each analysis. Finally, a stepwise CPA scheme was proposed based on the structure of SM4 cryptographic algorithm. The results of simulation experiments show that, on the premise of the success rate up to 90%, stepwise CPA reduces the demand of power traces by 25% compared to classic CPA. Field Programmable Gate Array (FPGA) based experiments indicate that the ability of stepwise CPA to recover the whole round key is very close to the limit of expanding the search space to the maximum. Stepwise CPA can reduce the interference of noise and improve the efficiency of analysis with a small amount of calculation.