To analyze the reason of crack caused at the variable section of the main girder, the finite element model of 260 t casting crane is built under 4 cases of a full load working cycle. The results show that the stress concentration at the crack (node 1) is much higher than in the mid-span structure where is usually high stress zone (node 2 and node 3 are selected to compare with node 1). Based on the stress amplitude method, these three modes’ fatigue lives and remaining lives are estimated. The results show that node 1 has the lowest life. For the girder to keep using, 3 improvement schemes are proposed for the stress concentration of the main girder, according to the structural characters of the main girder and the existing experience. One of them is to increase the web near to the main web, and two of them is to open holes in the stress concentration, and the last one is to replace the nearby main web plate with a thick plate. The reinforcement effect of each scheme is compared after stress simulation, and the last one is selected as the best one. It is provided favorable reference for structure to obtain high fatigue life.