The formation of soliton in the passively mode-locked fiber laser based on nonlinear polarization rotation technology is simulated numerically and investigated experimentally. The results show that due to the cavity pulse peak clamping effect, stable solitary waves can still be obtained in the laser which is described by the Maxwell-Bloch Equation. The higher density of the clamped soliton is, the wider frequency is. With the clamped density is increased, the soliton density is not increased, and the new soliton is formed.