In practical engineering, the cantilever beam with side beams, which is taken as an important bearing component, but constraints torque of side beams is often neglected, tends to unsafety. Through the theory of constraining moment and symmetric structure in symmetrical loading, the simplified calculation formula of the constraints torque was proposed，at the same time its validity and rationality was proved by examples so that it could be used in the actual engineering design. Through changing the ratio of linear stiffness beween side beam and cantilever beam, the calculation results indicated that the value decreased along with the parameter Φ increasing. So, the designer should be increase the size of side beam section as much as possible for decreasing the value of constraints torque.