A data writing method for a multi-level cell (MLC) NAND flash memory and a storage system and a controller using the same are provided. The flash memory includes a plurality of blocks. Each of the blocks includes a plurality of page addresses. The page addresses are categorized into a plurality of upper page addresses and a plurality of lower page addresses. The writing speed of the lower page addresses is faster than that of the upper page addresses. The data writing method includes receiving a writing command and data and writing the data into a page address. The page address is skipped when it is an upper page address and a corresponding lower page address stores a valid data written by a previous writing command. Thereby, the accuracy of the data written by the previous writing command is ensured when a programming error occurs to the flash memory.