A phenomenon experimentally emerging in super-continuum generation with a piece of photonic crystal fiber pumped by quasi-continuum wave is reported.According to the numerical simulation which contains using the gain spectrum theory of modulation instability and solving nonlinear Schrdinger equation,it is proved that this phenomenon is generated by modulation instability.After the results of the simulation and the experiment being contrasted and analyzed,it is found that the simulation result including two mentioned methods is in excellent agreement with the experiment result which the wavelength difference between the two spectrum lobes and the central wavelength are 27.08 nm and 32.72 nm respectively.