As a preliminary process of knowledge organizing and sharing, tacit knowledge acquisition is an essential task in knowledge management. However, there are still various interpretations of what tacit knowledge is and where tacit knowledge derives from, such as tacit knowledge is innate and ineffable, etc. This is a kind of Agnostics in Philosophy that tacit knowledge is considered unmanageable and uninterpretable. According to the principle of ON PRACTICE written by MAO TSE-TUNG, proposed is a new theory that tacit knowledge derives from practices accompanied with guidance of explicit knowledge. Tacit knowledge is highly abstracted which derives from integration of explicit knowledge and practical experiences, and which supports efficient reasoning and rapid retrieving. A computer simulation model is also given to imitate acquisition process of tacit knowledge.