Abstract: According to characteristic of the Raman gain curve,the Raman spectral region of interest was divided into a number of small frequency intervals.The Stocks spectrum of every frequency interval was pumped by those of higher frequency and pumped those of lower frequency.The effcet between them can be described by coupled differential equation.Solving the equationgroup of the entire frequency range, the Raman spectrum in different length fiber pumped by diff-
erent intersity be calculated and the threshold pump intensitybe obtained by study the calculated spectrum.

Key words: monomode optical fibers, nonlinear effect, stimulate Raman scattering