Sound insulation encasing is an effective device for noise reduction. The paper used the electric machine and reductor of oil sucking machine as an example, a controlling approach of installing sound insulation encasing has been found based on a analysis of noise characteristic, at the same time, the effectiveness of sound insulation encasing was analyzed, after establishing finite element models by using SYSNOISE software, and the deficiency of noise reducing effectiveness in local domain is shown. Effectiveness of noise reducing is better and meets the technical requirement, after amendment design, and this provides some reference to solve practical problem.