Abstract: In order to study the transmission properties of optical signal influenced by self-phase modulation (SPM) in semiconductor optical amplifier (SOA), based on the transmission equation, the generation principle of SPM was analyzed, the temporal and spectral characteristics of pulse transmitting through the SOA were obtained, the simulations are made separately to the pulse with no chirp and pulse with chirp. The result shows that SPM makes the shape of pulse decline and makes the spectrum broaden to lower frequency, and the degree of declination and broadening increases with growth of the gain of SOA, the sign of chirp coefficient determines whether the spectrum will be broadened to the high frequency or to the lower frequency.