A foldable musical instrument pedal device having a positioning element hung at a base board. The positioning element is drooped when the base board stays in air, and a first stop portion is swung downwardly with a support frame and across a second stop portion from an external side of the base board to an internal side of the base board. When the base board is placed on a ground, each second stop portion is propped out from the base board to block the first stop portion in an upward swinging direction of the support frame, so as to fix the support frame to the fixed base.