The invention provides an online particle monitoring system and method based on a beta ray method and a light scattering method. The system and method are used for precisely measuring the concentration of particles in smoke online in real time. The advantages of the beta ray method and the advantages of the light scattering method are combined, the reference standard of a beta ray method measuring result to a light scattering method measuring result is achieved through an internal reference method, and therefore the concentration of the particles in smoke is precisely measured online in real time. The measuring result is not influenced by the size, color, chemical constitution change and other characters of the particles, and the advantages of being high in precision, outputting figures in real time and the like are achieved.