The present invention relates to a detection method for a particle size of a polymer polyol. In the prior art, the detection method is not applicable for the polymer polyol, or the error of the detection result is big. To solve the technical problems in the prior art, the present invention provides the detection method for the particle size of the polymer polyol. The method comprises the following process steps: weighing a polymer polyol sample; diluting the sample through a solvent; filtering through a screen; placing the filtered particles and the screen in a vacuum drier to carry out drying; cooling the filtered particles and the screen to a room temperature after completing drying, then weighing and calculating the particle size of the sample. The method provided by the present invention is a physical separation method; with the method, the loss of the active ingredients of the polymer polyol during the separation process can be effectively prevented, the active ingredients of thepolymer polyol can be maximally preserved, the particles in the polymer polyol can be effectively separated from the sample through the physical separation, the elementary substances and a certain amount of the compounds can be removed, the detected particle size of the polymer polyol is accurate, and the precision of the detected particle size is high.