In order to improve the upper limit of particle velocity measurement for single-frame long-exposure imaging method, influence factors of upper limit for velocity measurement are theoretically analyzed and experimentally researched with backlit imaging single-frame long exposure method from the perspective of image grayscale. The results show that for backward illumination, higher particle velocity leads to higher gray value of particle image and more difficult to distinguish it. From the perspective of track recognition, the influencing factors mainly include the particle diameter, the particle transmittance and the intensity of light source. It is found that the upper limit of velocity measurement for the opaque disk particle with the diameter of 50 μm reaches 1250 m/s by using the backlit imaging single-frame long exposure method velocity measurement device when the intensity of the light source is high enough.