A picosecond pulse seed source with narrow bandwidth, low repetition rate and dispersionmanagement-free characteristics is studied. The intracavity pulse evolution is analyzed through numerical simulation. The influences of the cavity length and the fiber Bragg grating (FBG)′s bandwidth on the pulse stability and pulse duration are investigated. Experiment setup is established to carry out the experimental research on the effect of cavity length and pump power. When the cavity length is 24.1 m, a stable optical pulse with 86 ps pulse duration is obtained successfully, the bandwidth and the repetition rate of which are 0.04 nm and 4.3 MHz, respectively. When the pulse is amplified for coherent anti-Stokes Raman scattering(CARS) spectrum detection, the theoretical spectral resolution can be 0.3 cm-1.