The present invention discloses an automatic crankshaft homing device having speed adjusting function, included set up one resilient component, one cylinder, also one crankshaft and one cam of the same crankshaft on a base, between the cam and the resilient component set up one slider component, and one first rack set up on the slider component, both side of the cylinder set up with one cylinder rod and one flow adjustment valve, and one second rack set up on the cylinder rod, one gear set meshed in between the first rack and the second rack; the cam can follow crankshaft rotation and drive slider component move to the resilient component, to bring the first rack through gear set to drive the cylinder rod by the second rack, to make the cylinder through flow adjustment valve to suck the medium; the resilient component can drive slider component moving to the cam, to bring first rack through the gear set reverse drive the cylinder rod by the second rack, to make the cylinder through flow adjustment valve to discharge medium, flow adjustment valve can control the medium flow in cylinder when suction and discharge in accordance with precise adjust the speed when crankshaft automatic homing.