The invention discloses an iraser light source in the generation of an optical fiber type 3-5-micron continuous wave difference frequency and a method for achieving the same, wherein a pump light and a signal light adopt a wavelength sectional combining plan, a rare-earth-doped fiber laser with suitable wavelength is selected, a pump source adopts a ytterbium-doped fiber laser with a wave band of 1,060 nm and wave bands of over 1,100 nm, and the wave bands can be exchanged with each other. The signal light adopts an erbium-doped fiber laser with an S wave band, a C wave band and an L wave band, and the wave bands can be exchanged with each other. The ytterbium-doped fiber laser and the erbium-doped fiber laser emit the pump light and the signal light respectively, the pump light and the signal light are adjusted to be parallel with an optical axis of a crystal by a polarization controller respectively and are combined together by a fiber coupler, then a lens focusing system focuses the two light beams into a periodical polarization lithium niobate crystal; and by adjusting the length of the polarization cycle and the crystal temperature of the periodical polarization lithium niobate crystal, the pump light, the signal light and the difference frequency light meet a phase matching condition to finally achieve the infrared output in the generation of the difference frequency.