In order to study the stability behavior of steel tubular formwork support with coupler without X-bracing systematically, the semi-rigid character of the couplers and different geometric parameters under idealized conditions are considered, and non-linear finite element models of twelve steel tubular formwork supports with coupler without X-bracing are established to analyze the critical loads and failure modes. According to the results of the finite element analysis and the partially-sway single post stability theory, a simplified model and corresponding formula are proposed to calculate the bearing capacity of this kind of formwork supports. The prototypes of twelve formwork supports are tested, and the stability behaviors and failure modes are derived. The test results correspond well with FEM analysis, and both of them showed that the typical failure mode of steel tubular formwork support with coupler without X-bracing is the whole sway instability in short axis because its whole flexural stiffness is weaker compared with the long axis. Through the comparison of all the results, it is concluded that the simplified model and simple formula has a considerable accuracy to forecast the critical load of steel tubular formwork supports without X-bracing under uniform load, which should be adopted in the design standard.