Particle diameter analysis method based on traditional particle tracking velocimetry (PTV) technology can be used to detect particle diameter only by geometric imaging, and it is impossible to analyze particle images when diffraction imaging size is much larger than geometric imaging size. Based on the principles of diffraction imaging, a quantitative relationship model for determining image gray value and particle diameter of a micro-nano-scale tracer particle is proposed through the quantitative analysis of the whole physical process related to laser, particle scattered light, CCD signal, and image gray value and thus the shortcomings of PTV technology in particle diameter detection are addressed. Based on the PTV experimental system, the digital camera and software of Micro Vec V3 are used to complete the shooting of SiO₂ particle images, and the proposed model is used to analyze particle diameter from the captured images. The experimental results show that the proposed method has a high accuracy.