The invention discloses a nonlinear polarization rotation mode-locked pulse laser and a one-way mode-locked operation method, and relates to a pulse laser. According to the method, an optical fiber coupler is introduced into a nonlinear asymmetrical resonant cavity; a pulse in the laser in the clockwise direction is amplified by a ytterbium-doped fiber and reaches a polarization beam splitter through accumulated non-linearity effect of an optical fiber, so that saturable absorption is realized to realize mode locking; after a pulse in the counter-clockwise direction is amplified by the ytterbium-doped fiber, a certain amount of energy is output by the optical fiber coupler; and then the pulse reaches the polarization beam splitter; due to output loss of the optical fiber coupler in the counter-clockwise direction, the power is relatively low and the non-linearity effect is small; the laser power in the clockwise direction is relatively high; the accumulated non-linearity effect is relatively large; and after mode-locked operation is realized, enough non-linearity effect cannot be accumulated by weak spontaneous radiation in the reverse direction to realize mode locking, so that the energy of the laser is completely concentrated in the mode locking direction.