1. A data accessing method operated by a flash memory controller to access data from a plurality of physical page addresses of a plurality of physical blocks in at least a flash memory chip, comprising: providing a plurality of logical page addresses for being accessed by a host system; creating a logical page-to-physical page mapping table to record the physical page addresses mapping to the logical page addresses respectively; creating a physical page-to-logical page mapping table to record the logical page addresses mapping to the physical page addresses respectively; receiving a data to be written and a logical page address to be written from the host system, writing the data to be written into one of the physical page addresses, updating the logical page-to-physical page mapping table to record the logical page address to be written mapping to the physical page address, in which the data to be written is written, and updating the physical page-to-logical page mapping table to record the physical page address, in which the data to be written is written, mapping to the logical page address to be written; and comparing the logical page-to-physical page mapping table with the physical page-to-logical page mapping table to determine a plurality of valid physical page addresses and a plurality of invalid physical page addresses among the written physical page addresses.