Abstract: The hard X-ray free electron laser device demands that the current resolution of power supply is better than 2.5×10^-6. Therefore, the resolution of the direct current transformer (DCCT) must be higher than the above index, and reducing the modulation noise of DCCT is an important method to improve its resolution. In order to reduce the DCCT’s modulation noise, a mathematic model of modulation noise was proposed, an approximate expression of magnetic modulation noise was derived, and the relationship between modulation noise and related factor was analyzed in this paper. A method to evaluate the modulation noise was presented, which used the periodogram command of Matlab to obtain the noise power spectrum. The noise characteristics of Sinap-DCCT product were analyzed by using the mathematic model and evaluation method. The result shows that the relationship between modulation noise and related factor is consistent with mathematic model. According to this, the solution of reducing DCCT modulation noise was given.