It is difficult to obtain the accurate data of pure exhaust noise under common lab conditions because of interference of background noise. Two practical and effective methods have been proposed to measure the pure exhaust noise of engine in this paper. The experimental study on pure exhaust noise was carried on under the full-load speed characteristics curve of engine through using an acoustic enclosure designed by this paper. Test results indicate that experimental unit is right. The pure exhaust noise become stronger and the middle to high frequency noise increases with engine speed increasing. The pure exhaust noise of engine is coherent with background noise, and the value of which calculated by the theory of coherent acoustic is corresponded with the experimental data, so it is verified that the two measuring methods for pure exhaust noise of engine in this paper is feasible．