The structural design of the trackless telescopic gantry crane which depends on the traditional empirical methods tends to be conservative and material waste is serious. In order to take full advantage of the bearing capacity of its material, it is necessary to do the structure optimization analysis. During the optimization design of the trackless telescopic gantry crane, in view of the existing problems such as the non convergence and the low computation efficiency of the nonlinear contact model, a new optimization method which was used the node coupling model to replace the contact model was proposed to construct the response surface approximation model, and the model structural parameters were optimized by combining the approximation model and the multi-island genetic algorithm. Taking the component cross-section size of the trackless telescopic gantry crane as the initial design variables, the structural strength, stiffness, weight as the model response, the design method of optimal latin hypercube in the Isight platform was invoked to generate the design variable sample points, and then the finite element analysis software ANSYS was invoked by the DOE module to complete the sample point model simulation to screen out the design variables which had a high effect on the structural response. On this basis, the optimal design variables were used to construct the response surface approximation model, with the structural strength and structural stiffness as the constraints, the lightest structure weight of gantry crane as the objective, and the multi-island genetic algorithm was used to optimize the response surface model. The results of optimization indicated that the structure weight of gantry crane could reduce 23.4% in the premise of ensuring the structure performance of gantry crane, and the lightweight effect was obvious. The proposed optimization strategy can quickly obtain the global optimal solution and greatly reduce the computational quantities, which provides the theoretical basis for improvement of the trackless telescopic gantry crane structure.