In the paper, we analyse the effect of the sampling density on the measuring accuracy of the period of interference fringes by using a photodiode array of 1024 elements. We can get the highest measuring accuracy of about 2 × 10-5 when the sampling number of one fringe is from 10 to 80 and the sampling data quantization is 256. The loss the sampling data quantization is, the smaller the best sampling density will be.