The CGHAZ Microstructure has great effect on the toughness of welding joints and CTOD. This paper discussed the relationship between the CGHAZ and CTOD .The samples, carried out on high-strength steel EQ56, were observed by scanning electron microscopy. The Microstructural analysis result shows CGHAZ microstructure of EQ56-5, presenting high CTOD with 0.272mm, is low-carbon lath martensite and little M-A constituent, which contribute to the better toughness of EQ56-5. CGHAZ microstructure of EQ56-4, presenting the low CTOD with 0.082 mm, contains M-A constituent. The M-A constituent makes the CGHAZ brittle. The crack would grow easily along the M-A grain boundaries.