In this paper, the fast continuous weak Hash (FCWH) in strings is proposed and its theoretic and practical applications are investigated. First, FCWH is conceptualized and a uniform construction framework for FCWH is formulized from an algebraic viewpoint. Secondly, the theoretical and experimental collision probabilities of FCWH are analyzed, and the related work by Michael O. Rabin is generalized and strengthened. Finally, by generalizing the Karp-Rabin algorithm for string-matching problem, FCWH is applied to solve the problem of sequential extraction of common substrings (SECS), and, based on SECS, the express synchronization (X-Sync) protocol is designed to address the issue of real-time backup and the retrieval of multiple versions of a given document in the current environment of broadband communication network and cloud computing.