The present invention discloses a method for controlling flash timing of an extension flash module cooperating with a mobile device to provide supplemental light. The method includes: detecting a specific event before a flashable time period, wherein the flashable time is a period from the time at which a last photo sensor row of a specific frame begins exposure to the time at which a first photo sensor row in the specific frame ends exposure, wherein the time period from when the specific event occurs to a flashable time in the flashable time period has a fixed length of time; and triggering a flash instruction after a delay time from when the specific event occurs, such that the flash timing of the extension flash module is in the flashable time period.