The invention discloses a method for precisely measuring the radial internal clearance of a large-size bearing, which comprises the following steps of: making preparation before measurement, namely placing the bearing on a cushion block and fixing a fixing block on a bearing inner ring, and then fixing a magnetic frame on the fixing block; placing a clump weight at the center of the surface of a bearing outer ring, and placing the needle of the magnetic frame on the center of the clump weight; on the dial, reading a measurement value U1 measured when the bearing is freely placed and the othermeasurement value U2 measured when the bottom of the bearing outer ring leaves the cushion block; finally, obtaining the radial internal clearance |U1-U2| of the large-size bearing. The method makes full use of the current conditions to take the measurement according to the principle of radial internal clearance measurement. In the invention, required equipment is simple in structure, the method is easy to operate and has high measurement efficiency, and in addition, the method not only takes a precision measurement but also has low cost.