Abstract: Assembly steel strut system was applied to retain soil in deep excavation and combined multiple H steels jointed by high-strength bolts. The strut was usually supported by beams and columns at nearly the same interval. Practices showed that the strut capacity was always overestimated if the beam displacement was ignored. To simplify the question according to the actual mechanical condition, the beam and column were assumed to be springs and the strut was assumed to be a steel. A formula of the minimum beam stiffness for a single beam was provided based on the displacement continuous condition so that the calculation length of the strut in the horizontal plane equaled the interval of columns. Furthermore, the influence of factors such as the initial imperfection of structure, number of beams, distribution style of steel in the beam and steel elastic-plasticity on strut stability was discussed. With a deep excavation example, it was shown that the strut capacity based on the minimum beam stiffness and the monitor result were less than those based on material parameters. The capacity of the struts in this project was determined by the beam stiffness. The calculation length of strut in the horizontal plane should be the entire length of strut because the column stiffness was less than one-tenth of the beam stiffness.