The invention discloses a data writing method and system for a data mode log file system. The method comprises the steps that A, a log block weak fingerprint is calculated for a to-be written log block; B, whether or not there exists the same log block weak fingerprint in a fingerprint database is detected, if not, a new log block weak fingerprint record is added, wherein the log block weak fingerprint in A and a corresponding log block number are contained in the record, execution of C is conducted, and if yes, execution of C is conducted; C, physical writing of the to-be-written log block isconducted; D, a file block weak fingerprint for a to-be-written file is calculated; E, whether or not the count of the log block weak fingerprint which matches with the file block weak fingerprint inthe fingerprint database is 1 is detected, if yes, an address mapping table of the to-be-written file block is modified, a logical address is mapped to a physical address of the corresponding log block, and if not, physical writing of the to-be-written file block is conducted. The method has the advantages that repeated writing of the log file system can be greatly reduced to improve the performance of the log file system and prolong the service life of a solid state disk.