Abstract: The hard X-rays of solar flares are produced by Bremsstrahlung interaction ofhigh energy electron beam impinging on solar atmosphere. Based on a simple physical model of flare loop, and under the assumption that the high energy electronswith synchronous variation between the beam flux and spectrum index are injectedfrom the top of the loop, some calculated results about hard X-ray spectrum evolution with different atmosphere density are obtained in this paper. These resultsshow: in low density area, such as in the top of loop, the hard X-ray spectrum hassoft-hard--hard evolution property, in high density area, such as in the foot points ofloop, the spectrum possesses soft-hard-soft variation. The time duration of injectingelectrons will influence the height of region in the loop where the one type of hardX-ray spectrum evolution will transform into the other one.