To solve the cracking problem of the side apron plate generated in the underframe manufacture process of "Fuxing" EMU,the possible causes were analyzed. The primary and secondary causes of the cracking were confirmed by analysis.The structural design problem is the primary reason and the improper operation during the side apron plate installation process is the secondary reason. For those reasons,the welding procedure of the apron plate had been optimized and the quality of the weld had been increased. The bearing capacity of the weld had been increased 50% in theory. It is also concluded that the effective joint penetration should not be less than 3 mm in the aluminum alloy sheet welded structure,and complete penetration welding process should be used to ensure the bearing capacity of the welds.