The present invention relates to a non-contact automatic detection method for cast pipe socket roundness. The method comprises specific steps: S1, fixing is carried out; S2, two points are confirmed;S3, returning to the origin is carried out; S4, the actual central axis is determined; S5, operation is carried out; S6, the sectional drawing of a contour line is determined; and S7, the roundness value is determined. A laser ranging sensor is arranged at the tail end of a six-axis robot for conveniently sensing accurate data, thereby reducing measurement errors caused by manual participation; the six-axis robot is then driven to detect a group of data from outside to inside in a direction of the central axis of the cast pipe socket, and the operation is repeated for 12 times; and finally, the roundness value of each step of the cast pipe socket is obtained. An upper computer is used for processing, and the possibility of errors caused by excessive large calculation pressure caused by manual formula calculation is reduced.