Mechanism of steel output growth was resolved by two steps: firstly a standard mode was given under some assuming conditions, and its main character is steel output varies with the same speed as GDP. Steel output, steel products in use, GDP and their annual increasing rate were analyzed under standard mode. Some kinds of possibilities deviate from standard mode and their influence on steel output were discussed. Basic calculating formula for the annual increasing rate of steel output was obtained. Effect of deviating from standard mode on steel output per unit GDP was analyzed. Sixteen kinds of phenomena, which caused the annual increasing rate of steel output excessively exceeding GDP were put forward that extensively related to the social phenomena. We should make differences among these phenomena in macro regulation and control. Steel output per unit GDP in China has a large space to decrease based on the data of steel output per unit GDP among China, Japan and USA. A new train of thought of macro regulation and control for steel and iron industry in China was provided.