Abstract: The effect of postweld heat treatment on the microstructure of combined welded joint of PCrNi3Mo steel is studied. The transformation mechanism of charactcristic zone microstructure and embrittlement tendency of backing weld arc also discussed. The results show that the microstructure in backing weld bond become more complex and thincr carbon migration laycr was formed as large quantitics of type M₂₃C₆ carbides were prceipitated during the psotweld heat treatment.The embrittlement tendency of weld metal is relativcly little because the high crabo content in the bottom weld can play a certain part in restraining the transformation of sigma phase.