Based on Mie scattering theory, the formula for calculating mass concentration of spherical particles by optical particle counter was established. Considering non-spherical particles, starting from particles size distribution, the concept of the average mass was put forward, and the theoretical formula for calculating mass density of non-spherical particles was derived. Using this theoretical formula, the rationality of the calculation formula of optical particle counter for non-spherical particles was verified. The aerosol mass density algorithm based on average mass was proposed, and only two coefficients need to be calibrated in the algorithm. The experimental results agree well with the mass density values calculated by the algorithm, the slope of fitting line is 0.9713 and the correlation coefficient is 0.9998. This algorithm offers a feasible way for the optical particle counter to measure aerosol mass density in real time.