A flash memory device and a controlling method are provided. The flash memory device includes a memory array, an in-place update module, an out-of-place update module and a latency-aware module. The in-place update module is used for performing a program procedure or a garbage collection procedure via a bit erase operation or a page erase operation on the memory array. The out-of-place update module is used for performing the program procedure or the garbage collection procedure via a block erase operation or a migration operation on the memory array. The latency-aware module is used for determining a relationship between a first overhead of the in-place update module and a second overhead of the out-of-place update module.