Abstract: Geometric defects will significantly reduce the stability bearing capacity of a steel arch structure. In order to reasonably determine the unfavorable geometrical defects of a steel arch, the multiple response surfaces method based on subspace division is used to fit the bearing capacity function. Then, according to the Lagrange multiplier method of the extremum of the function, the geometric defects distribution with the lowest bearing capacity is solved. Taking the two hinged circular steel arches as examples, and considering the changes of parameters such as the load ratio and the number of deviation nodes, the mechanical properties are investigated for defective arch structures obtained by the proposed unfavorable defects method and the consistent mode imperfection method. The results show that:1) the multiple response surfaces method based on subspace is more accurate in fitting the bearing capacity function, and the prediction value of the bearing capacity is close to the result of the finite element analysis; 2) the geometric imperfections obtained by using the fitting function to calculate the extremum are more unfavorable to the bearing capacity, and within the parameters in analysis, the corresponding structural bearing capacity and initial stiffness is averagely about 10% and 22.8%, respectively, lower than that obtained by the consistent mode imperfection method; 3) with the increase of the number of deviation nodes, the differences are increasingly large between the distribution of unfavorable defects based on bearing capacity fitting and the distribution of the consistent defects, and the positive and negative deviations of nodes are more obvious for the unfavorable defects distribution based on bearing capacity fitting.