Abstract: The power arm of the excavator was welded from Q345B steel plates. After welding, cracks were found in a steel plate during inspection. Macroscopic and microscopic analysis, composition testing, mechanical property testing and metallographic structure analysis were carried out on the cracked steel plate. The results show that the main reason for the cracking of the steel plate is that the shrinkage porosity and inclusions formed in the smelting process of raw material caused the formation of large-size cracks and banded inclusions during rolling, leading to the cracking of the steel plate under welding stress. It is suggested that the formation of sulfide in raw material should be reduced by controlling the concentration of sulfur in steel and the cooling rate of liquid steel as well as adopting low temperature casting and electromagnetic stirring.