We experimentally report on the interference phenomenon between ultrashort pulses in fiber media. When the pulse interval goes further to a certain small scale, the interference effects can arise. As a result, the spectrum appears to be a modulation phenomenon. Different from the phenomenon of bound-state pulses in a fiber laser cavity, the relationship between the modulation period of the spectrum and the pulse interval in the time domain seems to go against the interference principle. At the same time, the time interval between the two pulses is far greater than the time difference caused by the optical path difference. We are very confused. It seems that when two pulses overlap by a certain proportion in time, they will push each other apart to increase the time interval. Moreover, when the pulse interval becomes smaller and the two pulses are overlapped in time, the corresponding autocorrelation trace exhibits only one peak with regular modulation instead of the conventional three peaks with equal spacing, which looks very strange and is hard to explain. Limited by our technical tools, we could not illustrate the physical causes, but there is no doubt that it will be very meaningful to research what happens. Our observations have found some novel optical phenomena in experiment, which may provide some reference for further exploration of interference phenomena between ultrashort pulses.