This paper focuses on the rapid charge transfer of lock-in pixels in time of flight 3D image sensors. Through the principal analysis, a mathematical model of potential distribution in a pinned photodiode (PPD) in different comb shapes is established. Based on this model, the influence of different comb shapes on the accelerating electric field in PPD is analyzed. The semiconductor device simulation tool SPECTRA is applied to verify the effectiveness of the model, and the simulation results are analyzed and discussed. When the width of comb tooth is in narrow and medium range, the potential changes more obviously with the increase of comb tooth angle α, whereas the potential becomes stable even if the comb tooth angle α increases sharply with the wide comb tooth width. The proposed mathematical model contributes to instructing the design of pixel transferring electrons rapidly and resolving image lag.