To solve the problem that rules in the existing context-aware systems are manually specified by developers or users, an automatic rule generation method is proposed. Context-Aware systems are regarded as decision systems, and context information are reduced with discernibility matrix so as to generate rules. Because the data that can be utilized are limited, the generated rules can not entirely cover the domains of contexts. The rule matches current context probably does not exist. A rule matching algorithm is presented as the solution to this problem. Finally, the efficiency and effectivity of the proposed method is verified.