Commercial off-the-shelf (COTS) components have been widely used in the aerospace at home and abroad, but there is no systematic theory of quantitative evaluation of the risk. Therefore, based on Shannon’s information entropy, the risk information entropy is proposed to describe the risk uncertainty of COTS components in space application. And a theoretical framework of COTS components space application is established, which includes calculation and decrease of the entropy. According to whether the failure mode and failure frequency ratio are known, different risk information entropy models are established for components. Based on this, a system risk information entropy model is established, and a continuous optimization method for the risk information entropy model is proposed. In addition, it introduces how to achieve system entropy reduction from several aspects, such as selection guidance, application certification, reliability reinforcement, and status detection.