Abstract: The concept of "weld bead heat affected zone" (WBHAZ)was proposed.The microstructure characteristic of multi-pass weld metal of 9%Cr-1%Mo high alloys heat resistant steel was analyzed and discussed by dividing characteristic zone in the WBHAZ and by plotting welding thermal cycles for characteristic points in the characteristic zone.The distance from the succeeding molten pool determines the peak temperature as well as the number of welding thermal cy cle which every characteristic point subjected to.The different peak temperature results in different phase transformation zone as well as different phase transformation.The different number of welding thermal cy cle also results in different phase transformation.The welding thermal cy cles is classified into several types, and according to the type, the weld metal microstructures can be interpreted thoroughly.This method is suitable to the other Cr-Mo high alloy steel weld metal as well.