This paper develops a new method for the calculation of low speed characteristic of compressor. Considering of the compressibility of the air, the low speed index of pump machinery similarity theory is improved. A new index calculation method is proposed, and the correction factor is added in the modified theoretical formula to increase the calculation accuracy of the compressor characteristics. Genetic algorithm is adopted to realize the optimization of the calculation to obtain the best value of the correction factor. It does not need to consider the structure and thermodynamic parameters of the compressor, and calculates the low speed characteristics of the compressor based on the existing characteristic curves. The calculation results show that the characteristic curves solved by the method are reasonable in changing trends.