The mode-locking mechanism of an all-polarization-maintaining fiber laser based on a nonlinear polarization loop mirror was demonstrated. Instead of a fiber coupler in the traditional nonlinear amplifying loop mirror based mode-locked fiber laser, a polarization beam splitter (PBS) was implied in the nonlinear polarization loop mirror. The combination of the PBS, non-reciprocal components and a piece of gain fiber acted as the core elements in nonlinear polarization loop mirror to achieve stable mode-locking in an all-polarization-maintaining fiber laser. A passively mode-locked erbium-doped fiber laser based on this mode-locking mechanism was also presented. The laser directly emited an optical pulse train of 141 fs duration (75 MHz) with 30 mW total output power. The laser had bidirectional outputs, and the power from two outputs was adjustable by adjusting the waveplate in the cavity. Moreover, the environment stability of the free-running laser was characterized, yielding an average output power fluctuation of <0.05% in 1 h and the relative repetition rate stability of 2.0×10-8 in 1 s. The configuration enables self-start mode-locking, generating optical pulse trains with high environmental stability, high repetition rate and short pulse duration and fully meets the requirements of ultra-short pulse laser sources in femtosecond based ranging, laser machining, spectroscopy and other applications in spaceflight technologies.