The present invention provides a method for accessing a flash memory module, wherein the method includes the steps of: writing data into a plurality of pages of a specific block, and establishes or updates a F2H mapping table based on physical addresses of the plurality of pages and logical addresses of the data; using the F2H mapping table to update a H2F mapping table; initializing a flush-bitmap, wherein the flush-bitmap records a plurality of flush bits corresponding to the physical addresses of the plurality of pages, respectively; receiving a trim command from a host device, wherein the trim command asks to mark at least one of the logical addresses of the data as invalid; updating the H2F mapping data according to the trim command; updating the flush-bitmap according to the trim command; and writing the updated H2F mapping table and the updated flush-bitmap into the flash memory module.