A method for adjusting over provisioning space and a flash device are provided. The flash device includes user storage space for storing user data and over provisioning space for garbage collection within the flash device. The flash device receives an operation instruction, and then performs an operation on user data stored in the user storage space according to the operation instruction. Further, the flash device identifies a changed size of user data after performing the operation. Based on the changed size of data, a target adjustment parameter is identified. Further, the flash device adjusts the capacity of the over provisioning space according to the target adjustment parameter. According to the method, the over provisioning ratio can be dynamically adjusted.