Based on the coupled Ginzburg-Landau equations and Jones matrix of all devices, a numerical model of an all-normal-dispersion mode-locked fiber laser by nonlinear polarization rotation is proposed. The polarization states of different points across the pulse are calculated along the cavity. It is found that when the linear birefringence of the fibers is strong, the evolution of polarization state in the fiber rounds a period of one beat length. Over one beat length, the state of polarization changes from right-handed elliptic to linear,left-handed elliptic, linear, and then back to right-handed elliptic. Different from a common saturable absorber, the modulation depth of the equivalent saturable absorber of the nonlinear polarization rotation varies with the wave plates angles. The dependence of modulation depth on wave plates angles is calculated. The results show that compared with the half wave plate and quarter wave plate before polarization beam splitter, the one after polarization beam splitter has a more obvious effect on modulation depth.