This paper presented a nonlinear analysis for Nakamura's model to investigate the influence of parameters, including the weight of pedestrians, the ratio of synchronized pedestrians, the function describing how pedestrians synchronize with the bridge natural frequency, etc. on lateral vibration amplitude of a footbridge. Theoretical analysis and numerical simulations indicate that the function describing how pedestrians synchronize with the bridge natural frequency may not be equal to 1.0 and can be far away from 1.0 even though the lateral vibration is excessive. The results are helpful for the application of Nakamura's model in engineering practice and can provide references for field measure.