The invention discloses a method for measuring the 
diameter of a non-contact 
forging on line by using 
laser. A pair of parallel linear guide rails arranged at equal altitude on the two sides of the 
forging is used in the method. The method is characterized in that: a 
laser transmitter is arranged on one guide rail, a 
laser receiver is arranged on the other guide rail, the 
laser transmitter and the laser 
receiver are driven to travel by two 
stepper motors respectively, the positions of the 
laser transmitter and the laser 
receiver are kept synchronous during the travel, the distance between the tangent point of the laser and the section circle of the 
forging can be computed quickly through the travel distance of the 
stepper motors and the connection and interruption of signals of the laser receiver, so that the 
diameter size of the forging can be obtained. The method has the advantages that: two mutually independent linear guide rails are adopted, so that the 
diameter of the forging such as a spindle of a 
windmill generator and metallurgic rollers in a wider range can be measured by selecting a proper guide rail length; measuring staff are not required to 
contact high-temperature environment, and manual measurement is avoided, so that the diameter of the current forging can be obtained quickly, precision is high, the shortcomings of long time and large error of the manual operation are overcome, and forging speed and forging precision are improved; the shortcoming of reserving relatively large cold 
processing allowance according to experience is effectively overcome, the allowance consumption of the forging is reduced, and the 
energy conservation and consumption reduction are realized.