Outofplane instability is usually one of the failures for planar tubular truss without outofplane braces under ultimate load. The mixed model of beam and shell elements, with the characteristics of high solving precision and less elements, can undertake overall stability analysis as well as the joints mechanic property when solving the issue of outofplane stability. Based on the principle of mixed finite element, nodes constraint equations at the interface between beam elements and shell elements were derived. And advices on the element division and solving method were proposed in order to avoid numerical difficulty in solving the mixed finite element equations. With the numerical solution and experimental test, it was found that the outofplane instability process can be expressed accurately with less timeconsuming by the proposed method and the proposed solution. 