A data storage device and a method for sharing memory of controller thereof are provided. The data storage device comprises a non-volatile memory and a controller, which is electrically coupled to the non-volatile memory and comprises an access interface, a redundant array of independent disks (RAID) error correcting code (ECC) engine and a central processing unit (CPU). The CPU has a first memory for storing temporary data, the RAID ECC engine has a second memory, and the controller maps the unused memory space of the second memory to the first memory to be virtualized as part of the first memory when the second memory is not fully used so that the CPU can also use the unused memory space of the second memory to store the temporary data.