The invention provides a data writing method for a flash memory. First, a write command, a write address, and write data are received from a host. When a total number of block pairs in the flash memory is equal to a threshold value, and execution of the write command increases the total number of block pairs, the write data is written to a data buffer block of the flash memory, and the write address is stored in an address storage table. A target block pair comprising a target mother block and a target child block is then selected from the block pairs for integration. The target mother block and the target child block are integrated into an integrated block during receiving intervals of a plurality of subsequent write commands. Finally, the write command is executed according to the write data stored in the data buffer block and the write address stored in the address storage table.