To improve measurement accuracy, the coherent noise of interferometric system should be reduced effectively. A theoretical model of signal-to-noise ratio (SNR) in interferometric system with ring source is proposed by means of statistical optics. The relations among parameters of ring source, coherent area of optical field and SNR of system are analyzed. By maximizing the objective function of SNR, optimization is employed to ring source and then the optimized parameters are controlled by computer generated holography. According to the theoretical model, the SNR of a given system depends on area of ring source, which is inverse proportion to coherent area of optical field. The parameters of ring source can be controlled by spectral width of conical wavefront, distance between rotating diffuser and focal plane, and spatial carrier frequency of computer generated hologram (CGH). Experimental comparison between point source and ring source shows that the average SNR of system increases from 5.2 dB to 8.6 dB by utilizing ring source.