Based on the background of application of modern laser industrial manufacturing, the diagnostic technology of laser beam is discussed. Starting from the definition of laser beam width, two different kinds of laser beam width definition are compared. The relationship between laser beam diagnostic technology and definition of laser beam width is described. Then, according to the development of national and international standards, the beam width, beam divergence angle measurement and calculation are presented. Focusing on the principle and equipment based on the principle of hollow needle, the method with purpose on engineering application is discussed. The analysis results show that the definition including power (or energy) defined percentage of the total power beam width is better suitable for engineering application and the beam diagnostic technology based on hollow needle combined with hyperbolic fitting method, and it will well define the lasers performance capability.