SPKI (Simple Public Key Infrastructure) can be used for access control. It emphasizes decentralization. SPKI certificate can be generated by any keyholder to empower to grant or delegate the authorization in question. The application should commit the SPKI certificate to the server and the server independently decides to offer service or not rather than deciding by Certificate Authority. SPKI certificate largely uses the public keys of entities instead of their names. The certificate of SPKI is applicable to the model of B to C electric commerce. This paper introduces the fundamental theories of SPKI, including the certificate's structure, the principle of 5-tuple reduction, SDSI name, the advantage of SPKI and so on. In the end, the paper instances the SPKI-based multi-channel payment system to illustrate the SPKI certificate.