A method based on laser difference detection for measuring suspended matter concentration is introduced in the paper. It can be used for the concentration measurement of suspended particles such as smog, dust and other aerosols. Based on the light scattering effect of the suspended particles, real-time monitoring of the suspended particles without changing their properties can be achieved. By means of a direct amplification of the differential signal of the test and reference light, fluctuations of the laser energy can be reduced. The measuring principles and methods are presented in detail. Through the study of the light decaying mechanism during laser propagating in the suspended particles, the relationship between the suspended matter concentration and light intensity attenuation is deduced. We also build a laser differential detection system for the measurement of the suspended matter concentration. By measuring the smog of known concentration, practical feasibility of the measurement method is verified.