In this study, the effects of pressure on the optical sensor in an on-line particle measurement system were investigated in detail based on the quantum theory and kinetic theory of gas. The results show that pressure alters the divergent light angle by altering the density and refractive index of the gas, so that the focused beam position is offset from the center of the aerosol tube; this causes the intensity per cross-section area of the optical measurement volume (OMV) to change, creating a significant deviation in the measurement result relative to the actual sample. Theoretical and experimental dynamic models were established based on the geometric optics theory and the geometric mathematical relation to analyze these variations. The results show that OMV size increases as pressure and divergent light angle increase. During an actual particle measurement process, the dynamic models proposed here can be used to effectively adjust and reconstruct the optical sensor, which can substantially improve the precision and accuracy of the measurement results.