An integrating mirror in common use was studied based on scalar diffraction theory and for arbitrary incidence laser beams,intensity distributions in different spatial planes behind an optical system are derived.Finally,simulated calculation examples are given and heat action of laser beams transformed by an integrating mirror is discussed.The results have shown that the transformed light beam has an obviously interferential and diffraction structure,and the distribution of incident laser beam and the choice of position of working plane have a great influence on the transformed light beam.For the material with a good thermal conductivity,the effects of the interferential and diffraction structure of laser beam can be ignored in the research of heat action;however,providing the calculating software to model integrating mirror system is important to improve its utilizable quality.