Taking into account the small linear loss of silicon-on-insulator(SOI) waveguide, conditions and gain spectra of modulation instability(MI) induced by combined effects of self-phase modulation and waveguide dispersions are investigated. The impacts of various parameters to gain spectra of MI are analyzed theoretically. Results show that strong MI takes place even in the existence of low light power. The MI peak gain is 2 to 3 orders of magnitude larger than that achieved in optical fibers with the same light power. The linear loss of waveguide impacts gain spectra of MI, even within ultra-short propagation distance. The peak gain, peak gain frequency and bandwidth of gain spectra decrease to 66.828%, 41.683% and 41.6879% of their maxima at propagation distance z = 5mm, respectively.