A data reading method, applied to a data storage device that includes a flash memory capable of operating in a SLC mode and a multi-level cell mode. The data reading method includes reading a page corresponding to a first word line of the flash memory in the SLC mode according to a read command of a host to obtain a first data segment, writing a predetermined data into a most-significant-bit page corresponding to the first word line in the multi-level cell mode when the first data segment has an error, and reading the page corresponding to the first word line in the SLC mode again to obtain a second data segment.