A novel physical model was proposed to describe silicon-on-insulator (SOI) waveguide Raman lasers by considering the spontaneous Raman scattering effect in cavity. Numerical results show that the proposed model can well describe small-signal output characteristics of SOI waveguide Raman lasers and realize rapid analysis, design and improvement on them. The SOI waveguide Raman lasers were analyzed using the model. Results indicate that the key to reach the laser thresholds in room temperature is a SOI waveguide with large Raman coefficient, low loss, and short effective free carrier lifetime. By optimizing the transverse geometric size, a high overall Raman gain can be achieved in SOI waveguides with short effective carrier lifetime, which can increase the output power and conversion efficiency of SOI waveguide Raman lasers.