This paper firstly introduces the worldwide-used COMPSYN software into the numerical simulation of seismic motions

and then makes a second exploitation of it with boxcar source time function replaced by bell function.Validity of modified code is confirmed by comparing it with analytical method in total space.By the comparison between spectrum characteristics of boxcar and bell function

it is pointed out that the boxcar function is rich in high frequencies

which goes against the seismic motion simulation.For the same observation

the seismic simulation results from COMPSYN code with two different source time functions show that the time-history simulated by boxcar function is featured by "ringing" and "serrate" phenomena

and it becomes worse as cut-off frequency grows

which affects the recognition of simulated seismogram

while the time-history simulated by bell function is quite smooth

which explains the rationality of bell function used in seismic motion simulation.