Abstract: Tri-directional beam string structure (TBSS) is a new type of beam string structure which is used in the new Guangzhou railway station. It is necessary to conduct stability analysis of this structure. Firstly the stochastic imperfection method was improved to make it more applicable to the structures mixed with grid shells and BSS. Then the imperfect stability of TBSS was studied with the improved stochastic imperfection method and the consistent imperfection method. The results show that improved stochastic imperfection method is suitable for the imperfect stability analysis of the structures mixed with grid shells and beam string structure. As the degree of imperfections becomes larger, the structure is more sensitive to the imperfections and the influence of the imper-fections on its stability behavior becomes more obvious. The eigenvalue buckling mode which is the closest to the collapse mode of the structure is the most disadvantageous eigenvalue buckling mode, and the stability capacity of the structure with imperfections base on this eigenvalue buckling mode is the lowest. However, the stability capacity of the structure with imperfections based on the nonlinear buckling mode is lower than that based on eigenvalue buckling mode, and it is very close to the results given by the improved stochastic imperfection method.