Efficient space trimming of data storage device is shown, which uses a controller to program a trimming tag that corresponds to a trimming command issued by a host into a cache area of a temporary storage device. The trimming tag, therefore, can be flushed from the cache area to the non-volatile memory for non-volatile storage. The update of the host-to-device mapping table based on the trimming tag can be postponed to the moment that the controller is free.