A fast tool changing method with pre-unclamping step for controlling operations of the spindle, tool unclamping cylinder, and tool changing arm. The spindle has one end clamping the tool at the clamping position. The method includes following steps. In a tool pre-unclamping step, the clamp end of the spindle moves the used tool to a pre-unclamping position; the tool unclamping cylinder pushes the used tool to leave a taper of the spindle. In a tool changing step, the tool changing arm rotates to clamp the used tool and pulls the used tool away from the clamp end, and further aligns a target tool with the clamp end. In a tool inserting step, the tool changing arm inserts the target tool into the clamp end, to be clamped therein. Therefore, the tool unclamping and tool clamping operations are accelerated.