Abstract: Web data grows quickly in the short time，previous FTS may be changed when the sequence database is updated.Re-finding FTS will consume too much execution time and storage space.In this paper，an improved extended lattice，IE-LATTICE is designed to store the previous mining results.An efficient algorithm based on bidirectional constraint，called IM-FTS is proposed，which utilizes the previous results and constraint strategy to discover the new FTS just from the added and deleted part of the database.The analysis and experiments show that IM-FTS algorithm efficiently reduces the average execution time and storage space for mining FTS.