We study the effect of Coulomb potential on high-order harmonic generation (HHG) numerically and analytically. We focus on the influence of Coulomb potential on emission times of HHG associated with specific electron trajectories. By using a numerical procedure based on numerical solution of time-dependent Schr{o}dinger equation (TDSE) in three dimensions, we extract the HHG emission times both for long and short electron trajectories. We compare TDSE predictions with those of a Coulomb-modified model arising from strong-field approximation (SFA). We show that the Coulomb effect induces earlier HHG emission times than those predicted by the general SFA model without considering the Coulomb potential. In particular, this effect influences differently on long and short electron trajectories and is more remarkable for low-energy harmonics than high ones. It also changes the HHG amplitudes for long and short electron trajectories. We validate our discussions with diverse laser parameters and forms of Coulomb potential. Our results strongly support a four-step model of HHG.
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