Abstract: In order to achieve prediction and warning of mine coal-dust explosion, taking photodiode as light source, we designed an on-line test system for coal-dust particle size and concentration by narrow beam of light transmission technology based on extinction statistical method. We established coal dust testing model by extinction statistical method, deduced the test equation of extinction statistical method, and designed experimental apparatus and signal acquisition circuit for the testing system. We carried out coal dust measurement experiment for 0.05 mm, 0.24 mm, 0.72 mm and 1 mm four kinds of thickness of the beam diameter. Experimental results show that the standard deviation is 7.6% when the thickness of beam measurement particle size chooses 240 um and the designed measurement system can meet the monitoring requirement of mine coal dust, which verifies practicability and feasibility of the system.