We have implemented monolithic narrow linewidth single-mode single-frequency pulsed fiber lasers in master oscillator and power amplifier (MOPA) configuration based on highly Er/Yb co-doped phosphate fiber with core size of 25 μm. The narrow linewidth pulsed fiber laser seed has been achieved by directly modulating single-frequency CW fiber laser. An arbitrary waveform generator (AWG) was used to pre-shape the pulse shapes in order to avoid the pulse distortion and the dynamic gain saturation in the cascade fiber amplifiers. Based on the newly developed large core single-mode highly Er/Yb co-doped phosphate fiber in the power amplifier stage, the peak power of single-mode pulses can be scaled to more than 100 kW with transform-limited linewidth and diffraction-limited beam quality. These high power narrow linewidth single-mode fiber laser pulses have been successfully used to generate coherent THz waves based on difference-frequency generation (DFG) in GaSe crystal. The single-pass generated THz peak power can reach 0.12 W. Moreover, we have observed the external cavity enhancement of DFG THz generation by using ZnGeP2 for the first time, and implemented a high spectral resolution THz spectrometer based on the developed fiber-based tunable narrow linewidth THz source.