Abstract: Through the optimization of welding process and welding structure, the quality of butt joint of composite plate is compared and evaluated. By means of metallographic analysis, hardness test, X-ray energy spectrum, fracture analysis and other means, the cause of welding crack at the root area of (Q345R + S30403) composite plate is studied. The research shows that the dilution effect of stainless steel base material on root weld leads to significant change of weld composition and increase of weld hardening tendency, which is the root cause of hydrogen induced delayed crack. The effective control method of welding crack of composite plate was obtained, including 5 mm root face, unilateral V-groove, gapless pairing, etc, and the welding process of back root weld cleaning treatment and stainless steel back capping, which effectively avoided the occurrence of cold crack in the root. The research has important engineering guiding significance for the improvement of welding quality of anticorrosive structures such as petroleum, chemical, power construction and pressure vessels.