Laser particle size analyzer measures particle size based on the principle of light scattering. The scattering phenomenon of light can be strictly described by Mie theory, but the numerical calculation of Mie theory is very complex. The traditional view is that when particles are much larger than the wavelength of light (>2 µ m), their scattering can be described using the relatively simple Fraunhoff diffraction theory. The author of this article found through experiments that when using diffraction theory to analyze the scattered light energy data of large particles, a particle size distribution peak will appear "out of thin air" around 1 µ m. This article first describes and analyzes the above phenomenon, and then explains it using optical theory, confirming that it is caused by errors in diffraction theory. Later, it points out that diffraction theory can only be used in laser particle size analyzers when the refractive index of particles has an imaginary part, that is, when particles have absorption.