对菲涅耳双棱镜干涉测钠光波长实验中应用二次成像法测量双虚光源间距的相对误差进行分析，指出小像的测量误差是影响测量双虚光源间距的主要因素，提出改用当虚光源位于透镜前2 倍焦距时物像等大的测量方法，可较为准确的测量双虚光源间距和双虚光源到测微目镜的分划板的距离.

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.