In this paper，we analyze the relative error of the distance between two virtual light sources by secondary imaging method in Fresnel biprism interference，derive that the measurement error of the smaller image is a major factor，and put forward a more accurate method by measuring image size when the slit is located 2f in front of the lens to measure the distance of two virtual light sources and the inference distance between the virtual light sources and the micrometer eyepiece reticle.