The generalized stress and strength distribution interference theory is introduced to establish the reliability model of torsion spring in multi-failure modes. Using it as constraint condition, the lightweight optimization model of torsion spring based on reliability constraint in multi-failure modes is presented. And then an optimization design for a given structure of torsion spring is made to use the theoretical model under operating conditions and the reliability constraint. The weight of the opti-mized torsion spring is lighter than that of non-optimization. This verified the validity of the opti-mization method. The optimization method may contribution to design of aerospace equipments which demand strictly in weight.