The invention discloses an on-line measuring system for granularity of granules. The system includes a zero gas generator, a sampling device, a mixing tube, a measuring device, a vacuum pump, a separation device and a sample collecting device. The measuring device includes a shell, in which the outlet of the mixing tube is opposite to the inlet of the separation device. A laser generation device and a photoelectric sensor are arranged correspondingly on two sides inside the shell. When a gas-solid mixture enters the shell, an alignment laser, generated by the laser generation device, is blocked by solid granules, so that a part of the alignment laser is scattered, scattering angel being relative to the granularity of the solid granules; and the scattered laser is interfered with non-scattered alignment laser. The photoelectric sensor then transmits an interference image to a computer so as to calculate the granularity distribution of the solid granules. The system is based on laser granularity measurement principle and combines negative-pressure suction sampling measurement and intermittence sample returning technologies, so that the system has accurate results in time, and can achieve long-time on-line measurement on the granularity distribution of the solid granules in an industrial site.