The utility model provides a steel body diameter measuring device, include: two range finding light screen sensor of the orientation transportation steel that the chain the transports body lie in both sides and crisscross setting the that the steel body transported the chain respectively, every the range finding light screen sensor in the transportation chain the opposite side set up respectively one with the just right baffle of this range finding light screen sensor. By last, two light act distance measuring sensor bring -up test laser are by steel body reflection back, and first, two light act distance measuring sensor detect the point of 180 degrees within ranges on tapping sinistron, the right side semicircle, judge the distance of light -emitting curtain range finding sensing and steel body semicircle according to transmission and the time difference of receiving from this to the realization is compared in prior art to accurate detection of steel body diameter, and whole round of profile to the steel body that can be quick carries out the accuracy and detect. Compare in prior art, if the steel body through two light act distance measuring sensor alright carry out accuracy detection with quick whole round of profile to the steel body.