Based on drift-diffusion approximation and under axis-symmetric assumption, a two-dimensional(2D) fluid model is established for the plasma in the chamber of electron cyclotron resonance plasma source. A finite difference method is used for self-consistent numerically simulating the model. Numerical results of uniformity evolution of plasma density are obtained. From the analysis of the numerical results, the effects of background gas pressure, microwave power and current in magnetic field coil on uniformity of the plasma density are studied. The results shows that during the initial ionization, the uniformity of electron density is better than that of ion density. During the later ionization, the uniformity of ion density is better than that of electron density. As background gas pressure increases, the uniformities of both electron and ion densities increase, and the uniformity of ion density increases faster. As microwave power increases, the uniformities of both electron and ion densities increase with almost the same rates. As current in magnetic field coil increases, the uniformities of both electron and ion densities increase at almost the same rates. However, when the current in magnetic field coil becomes big enough, the uniformities of both electron and ion densities decrease at almost of same rates.