The invention provides a data access method of a flash memory. First, a write command, a write address, and target data are received from a host. A target block corresponding to the write address is then determined from the flash memory. Whether a storage space corresponding to the write address in the target block has stored data therein is then determined. When the storage space of the target block does not have stored data therein, the target data is written into the storage space of the target block. When the storage space of the target block does have stored data therein, whether a child block mapped to the target block exists in the flash memory is determined. When the child block exists in the flash memory, the target data is written into the child block.