The studies on Speech Acts of Directives (and the traditional study of Imperative Sentences) of local scholars have achieved some valuable results, including the definition and the inner classification of Speech Acts of Directives based on the speaker’s authority and power, the tone of sentence. However, it is observed that in speech communication, a directive sentence is given by the speaker based on some other influences besides those factors, which is against the former studies. Therefore, there should be a more complicated mechanism in the formation of a speech act of directives, not just formed by an individual factor. This article examines the selection process of the directive speech acts based on the theory of prototypic categories, and take more complicated, organic organized contexts into consideration. The context factors can be divided into constant factors and changeable factors. The combination of the two factors decides the range of selection of directive speech acts and the final form of output of the speech act.