The reception of laser pulses is one of the key technologies in the laser rangefinding. The stable and effective detection and the increased ability of anti-jamming are the main problems to be solved. In this paper, an elementary study on walk error in pulsed laser range finding is carried out. The photodiode response time is affected by the change of depletion layer width, which is caused by the interaction between depletion layer charges and photo-induced carriers. The basic theoretic model is set up to explain this process, and two characteristic parameters unit optical power sensitivity K and unit time jitter Δτp0 are introduced. The approximate relation between the photodiode response time, its jitter and the laser pulse power projected on the detector is obtained respectively, which are successfully employed to explain the walk error phenomena in the self-triggering pulsed time-of-flight laser rangefinding experiment.