Abstract: The multi-storey and multi-span plane frame was no obvious out-plane support because of some floors removed, and this lead to a multi-column buckle in the out-plane direction of the frame due to lack of lateral supports. The out-plane buckling behavior of two storey plane multi-columns subjected an axial load was investigated theoretically and its design criterion was presented in this study. Firstly, the out-plane bracing stiffness of columns in a multi-column model was obtained by theoretical derivation, so that an up-down column with a mid spring support could be treated as an equivalent analytical model which can reflect the behavior of the two storey multi-column model. Afterwards, the elastic buckling load and the corresponding effective slenderness of the multi-column was obtained by using the conclusion of the equivalent model. In the end, the ultimate load-carrying capacity of multi-columns was conducted by using the finite element numerical analysis, in which the random out-plane imperfections distribution of different columns in the multi-column model was taken account. From the study above, it was proved to be available that the design approach of the equivalent model could be used in a two storey multi-column model.