Since the birth of software engineering, it always are recognized as one pure engineering subject, therefore, the foundational scientific problems are not paid much attention. This paper proposes that Requirements Analysis, the kernel process of software engineering, can be modeled based on the concept of "common knowledge". Such a model would make us understand the nature of this process. This paper utilizes the formal language as the tool to characterize the "common knowledge"-based Requirements Analysis model, and theoretically proves that : 1) the precondition of success of software projects regardless of cost would be that the participants in a software project have fully known the requirement specification, if the participants do not understand the meaning of the other participants; 2) the precondition of success of software projects regardless of cost would be that the union set of knowledge of basic facts of the participants in a software project can fully cover the requirement specification, if the participants can always understand the meaning of the other participants. These two theorems may have potential meanings to propose new software engineering methodology.