This paper describes a semiconductor laser as the light source, Establish a real-time measurements optical system, with an particle number concentration of dust particles in the air. Gives a detailed structural principle of the system and how to use an optical sensor. According to Mie scattering theory, the light scattering characteristics of the response of the optical system. Experimental results show that the system has a high count accuracy, repeatability and counting efficiency for atmospheric PM2.5 monitoring.