This study presents an online size measurement method for burning particles based on the Fraunhofer diffraction theory. According to the spectral characteristics of medicinal strip combustion flame radiation in solid propellants, a blue-violet laser with a wavelength of 450 nm is used as the light source, and a filtering detection method with a passing band of 450 nm is used to eliminate self-luminous radiation influence of solid propellant combustion. An online measurement system for measuring the particle sizes of burning particles in solid propellants is developed. For calibrating this measurement system, standard particles with size of 10。9 μm, 160 μm, and mixed standard particles are measured. The combustion experimental results show that the size of burning particles in solid propellants are approximately 10 μm and 160 μm, presenting a bimodal distribution, and the number distribution is mainly concentrated around 160 μm. As the combustion time increases, the number of particles with a size of 10 μm increases and the number of particles with a size of around 160 μm decreases. For a solid propellant with same burning rate, the higher the initial measurement height, the smaller the combustion particle size. For measurements with same initial heights, there is no considerable difference in the average particle size of burning particles at different burning rates. This study provides a reference for the investigation of the solid propellant combustion process.