Based on Mie scattering theory, the formula to calculate mass concentration of spherical particles by optical particle counter was established. Considering non-spherical particles, starting from particle size distribution,the concept of the number of equivalent spherical particles was put forward, and the theory formula to calculate mass concentration of non-spherical particles was derived. The validity of the formula for non-spherical particles was verified. On this basis, aerosol mass concentration algorithm based on the number of equivalent spherical particles was proposed. The experimental results indicate that the mass concentrations calculated by the algorithm agree well with the actual values,so this algorithm offers a fessible way for the optical particle counter to measure aerosol mass concentration in real time.