The present invention discloses a high-peak-power single-frequency narrow-linewidth nanosecond fiber laser based on a triangular pulse, wherein the laser includes: pulsed laser generated by the laser seed injecting into a first power pre-amplifier through a first isolator, and then injecting into a second pre-amplifier and then injecting into a power amplifier; wherein triangle-shaped pulsed laser with fast rising edge is obtained by using electro-optic and acousto-optic modulator to modulate continuous wave single-frequency laser or a single-frequency semiconductor laser directly modulated by radio frequency signal; single-frequency triangle-shaped pulsed laser is employed as the laser source according to the characteristics of narrow intrinsic linewidth and suppression of linewidth broadening caused by SPM, and the power of pulsed laser is amplified through the MOPA system.