In laser materials processing, the transverse beam intensity distribution affects processing quality greatly. In this paper the phase shift (also called accessary phase overtaking) in laser beam is used to analyse the high power CO2 laser beam transverse size and intensity distribution. The transverse beam size and intensity distribution in different positions in a TLF6000 laser beam propagation were measure by UFF100. The theory is agreed with measurement. According to this theory, an optical transformation system is designed to attain the minimum laser beam intensity distribution near the laser beam focus.