The invention discloses an external floating top vertical metal can volume measuring method. According to the method, firstly, complete three-dimensional point cloud data are obtained through adopting a multi-station scanning and splicing mode at the upper and lower parts of a floating disc; and then, the integral thought is utilized, the can wall point cloud data are cut and are divided into a plurality of small cylinders, and the volumes of the small cylinders are calculated and are superimposed for obtaining the total volume. Through fitting the point cloud coordinate data of each small cylinder at the height parts, the girth sheet radius is obtained, and further, the area of the bottom of the cylinder is calculated; the cylinder volume is calculated according to the obtained height; and meanwhile, the temperature and gradient correction is introduced. The bottom volume calculation adopts an equal-area circle dividing mode, the volume of each round table body and each middle cone is calculated, and the obtained total displacement is superimposed. The method has the advantages that the operation is simple, the safety is high, the work intensity is low, the fast measurement of large-sized external floating top vertical metal cans can be realized, the data processing is convenient, the accuracy is high, meanwhile, the metrological verification cost is greatly reduced, and certain economic benefits are brought.