In the paper, the effects of composition adjustment on mechanical properties and production costs of steel are systematically studied through optimizing the composition control range of such strengthening elements as Mn, Ti and N as well as adopting staged industrial tests and regulating methods of closed-loop characteristics aiming at such problems as higher strength of raw materials in hot rolling, more roll consumption of pickling line tandem cold mill, easy slipping of rolling mill, large fluctuation of yield strength and higher costs in the production of S350GD+ZAM and S450GD+ZAM Zn-Al-Mg structural steels. The results showed that the mechanical properties of both products could meet standard requirements after optimization, rate of reaching the standard for properties of S350GD+ZAM was increased from 96.4% to 98.7%, allowance of yield strength for S450GD+ZAM was sufficient and its rate of reaching the standard was 98.2%; accumulated cost reduction of them in 2024 was RMB 749,320 Yuan so that production bottleneck was effectively broken as well as cost decreasing and benefit increasing were realized, which could provide reproducible industrial technical path for low-cost and stable productions of structural steel with Zn-Al-Mg coating.