According to the national standard GB/T1591-2018 "Low-alloy High-strength Steel" and the examples of steel structure construction, this paper analyzes the essential difference between Q355 and Q345 based upon the chemical composition of the steel. It further expounds upon the characteristics of Q355 low carbon, micro-alloy, pure purification and fine grain with the concept of carbon equivalent, which illustrates the objective fact that Q355 is a typically new type of fine grain steel. Based on the analysis, this paper only considers that the strength and grain refinement of the welded joints of high-strength steel are directly related to the micro-alloyed elements of steel. The welding thermal cycle will cause the loss of alloying elements. Therefore, the loss of alloying elements will increase after one more thermal cycle, which is unfavorable to the quality of welded joint and will inevitably reduce the comprehensive properties of the welded joint. This is the key to welding Q355 steel.