Laser particle size analyzer is an instrument based on the principle of static light scattering used to measure particle size. Herein, an ideal optical model of the laser particle size analyzer, which can measure scattered light in the range of 0?π, is proposed and its lower limit of measurement and resolving power are studied. Moreover, the optimal detector density coefficient of the ideal model based on the sampling law is discussed. Experimental results show that when the tolerance limit of scattered light energy distribution loss is 0.1%, the optimal detector density coefficient of the ideal model is 1.0905. When the noise intensity is 0.5%, the lower limit of measurement of the ideal model is 40 nm, and the resolving power for particles with the sizes of 100, 200, and 300 nm is 50.5%, 67.3%, and 75.0%, respectively. Conversely, when the ideal model is used to measure standard polystyrene microspheres, the lower limit of measurement can reach 50 nm, and the resolving power of particles with the sizes of 100, 200, and 300 nm is 50.0%, 55.7%, and 75.0%, respectively.