Based on the rigid body rule, an improved plastic hinge model is adopted to produce a spatial elasto-plastic beam element for the stability analysis of the steel dome structure of LNG storage tank by the consideration of the nonlinearity of geometry and materials. By using the load disturbing and the mode disturbing method, the critical load of the steel dome are obtained. The results showed that the stability of the steel dome is sensitive to the structural defects. Compared with that of the idealistic dome structure, the critical loads of the defected dome structure decreased by 60%. On the condition of disturbing load, the initial deformation of the whole structure is tiny, and the structural critical load is in-sensitive to the defect location. However, by assuming the initial deformation according to the structural basic mode, the whole deformation of the dome is larger, and the load displacement curve is smooth. With the increase of defects, the decline rate of bearing capacity of the dome is slowed down. By the comparison with ABAQUS, the stiffness matrix of the present elasto-plastic rigid body rule element is simple in derivation, clarity in physical meaning and linearity in form. The efficiency and accuracy of the proposed method in the nonlinear analysis for practical engineering is also proved with the advantage of dealing with the local defects of structure.