Based on the principle of virtual equivalent gain, the evolution of self-similar pulses in a normal dispersion-decreasing fiber is investigated. The occurrence of wave breaking during the process under highly nonlinear conditions and the effect of its revealed four-wave mixing on the process are also analyzed. The results indicate that the pulse spectrum broadening in the initial stage is dominated by self-phase modulation while by four-wave mixing in the later stage. The more intense nonlinearity is, the faster the pulse evolution converges to the self-similarity synchronously in both the time domain and the spectrum domain.