Thermal simulating technology was used to simulate weld CGHAZ of microalloyed steel with different thermal cycle for the purpose of investigation on morphology of M-A constituent and its influence on toughness. The experimental results showed that in comparison with base metal specimens after thermal cycle have poorer toughness for its larger size, elongated and sharp massive shape and non-uniform distribution. Toughness of specimen value has maximum value with maximum area fraction for cooling time of 7s, and then it will drop whether cooling time is longer or shorter. Moreover, its mean chords of all specimens are all smaller than 0.5μm. Only those with the length larger than 2μm or the length-width ratio exceeding 4, cleavage fracture can occur. So it is concluded that M-A constituent is not the main influencing factor of impairing toughness for steel with different thermal cycle for its smaller mean chord and area fraction.