We investigated the photon-number-resolving (PNR) performance of the InGaAs/InP avalanche photodiode (APD) as a function of the electric gate width and the photon arrival time. The optimal electric gate width was around 1 ns for PNR measurements in our experiment, which provided a PNR capability up to three photons per pulse when the detection efficiency was ~20%. And the dependence of the PNR performance on the arrival time of the photons showed that the photon number could be better resolved if the photons arrived on the rising edge of the electric gate than on the falling edge. In addition, we found that with the increase of the electric gate width, PNR performance got worse. The observation would be helpful for improving the PNR performance of the InGaAs/InP APD in the gated mode.