The invention discloses a monocular vision locating method applied to underwater robot. The locating method comprises the steps: firstly, distributing a marker, then optimizing and calculating magnification times mag according to collected images, obtaining a plurality of sample data by repeated collecting and establishing a functional relationship of the magnification times mag with a course angle psi and a proportion value dd; in actual application, calculating magnification times according to the course angle and the proportion value dd calculated out by the actual collected images and calculating out coordinates of the robot by a geometric method. Under the situation of guaranteeing higher locating precision, the method greatly reduces complexity of a monocular vision locating method and improves timeliness; thus, the method can be applied to a system without an accurate mathematic model or a system with a higher requirement on real timeliness or a hardware platform with low cost and limited processing performance.