An estimation method for read and write access performance adapted for a computer system including a host and a flash memory device is disclosed. The flash memory device includes a controller, and a flash memory module having a plurality of blocks. When the host transmits a writing performance query instruction, the controller executes a writing performance estimation process including determining whether at least one free block can accommodate data to be written. If Yes, a reply signal including an optimal writing performance is transmitted to the host; and if No, at least one block having data stored therein is selected for execution of a garbage collection process. When the host transmits a reading performance query instruction, the controller executes a reading performance estimation process including determining a threshold interval within which an average retry times of shift read is located to determine a corresponding estimated reading performance.