By using the designed photonic crystal fiber filled with argon gas,the effect of gas pressure on modulation instability(MI)gain is analyzed in detail.The MI gain bandwidth increases gradually as the argon gas pressure rises from 1P0 to 400P0(P0 is one standard atmosphere),while its gain amplitude slightly decreases.Moreover,the increase of the incident light power also results in the increase of MI gain bandwidth in the Stokes or anti-Stokes region when the incident power increases from 1 W to 200 W.Making use of the optimal parameters including the higher argon gas pressure(400P0)and the incident light power(200 W),we finally obtain a 100 nm broadband MI gain.These results indicate that controlling the MI gain characteristic by changing the argon gas pressure in PCF is an effective way when the incident light source is not easy to satisfy the requirement of practical application.This method of controlling MI gain can be used in optical communication and laser shaping.