This paper analyzes theoretically various nonlinear phenomena caused by focusing a laser beam into high-pressure gas, and the interaction and interference produced, measures the relationship among output energy, input energy, gas pressure and focal length when the fourth harmonic Nd:YAG laser is focused into the Raman cell filled with methane. Experimental results are in agreement with theoretical ones qualitatively. Based on these we can optimize and use the stimulated Raman scattering.