Based on Mie scattering principle, a method for PM2.5 concentration detection was designed by combining photometer and particle counting, to some extent reducing the interference caused by the particles with the diameter equivalent to or larger than 2.5μm. It uses 655nm laser to irradiate on suspended particles, receives Mie scattering light intensity through the photodiode, and applies the centroid method to make statistical analysis on the light intensity data with the maximum probability, and then use it as the real scattering data generated by PM2.5 particles to deduce the distribution of PM2.5 mass concentration. Finally, compared with the PM2.5 detector produced by TSI company in the same environment, it is shown that using the proposed new method, the measurement error is significantly reduced, and it is reduced by only ±5μg/cm3 at high concentration of PM2.5.