Structural optimal design is traditionally performed according to the elastic limit rule which makes the structure overweighted or strength over conservative to an extent. Shakedown theory is implemented in the present study in order to measure load-capacity performance of structure. It can determine the strength of a structure under arbitrary varying loads without loading history where failure form of incremental collapse, ratcheting and alternate plasticity will be avoided. The current research subsequently provides a parameter optimal design scheme with respect to maximum structure strength and strength-to-weight performance in the framework of shakedown which makes the optimal structure design in a relatively practical way. With this motivation, the formula of proposed shakedown limited parameter optimization problem is derived and solved based on genetic-gradient coupling algorithm. The present coupling algorithm is essentially solve a double-loop problem where the inner loop adopt interior point method to solve shakedown problem under a given parameter combination while the outer loop use genetic algorithm to find the optimal parameters based on a given shakedown fitness. In addition, external database technique is applied to accelerate computation and prevent unexpected interrupt. Subsequently, a test example which is a optimal parameter design problem of plate with a circular hole is presented to demonstrate the accuracy and effectiveness of the proposed method and algorithm. In addition, the proposed method is also utilized to determine the optimal load-bearing capacity of a airtight module to be used in a manned spacecraft. Moreover, optimal shakedown load design results of the manned airtight module as well as optimal strength-to-weight efficiency design results are given in the end. This study confirmed that genetic-gradient coupling algorithm is a effective means for determine the optimal parameter in accordance to the shakedown load domain.