The invention relates to the field of non-contact detection of circularity values of inner circles of pipelines, in particular to an axis calibration and circularity detection method for a bellmouth of a cast tube. The method comprises the steps of S1, obtaining two points; S2, obtaining data; S3, generating a circularity value; S4, judging the circularity value of each step, wherein a laser distance measuring sensor arranged at the tail end of a six-axis robot generates a polygon similar to a circle along the bellmouth of the cast tube through processing of program software, and the accuracyof the processed circularity value is high. According to the steps of the method, the six-axis robot carries the laser distance measuring sensor to move to a selected point of another step to repeat detection action, the circularity values of all the steps are judged sequentially, the manual participation in the calculation and measurement is reduced, and the precision is improved.