The invention relates to a flow bypass device and method with the adjustable swing frequency and belongs to the field of hydrodynamics. The flow bypass device is characterized by comprising a flow bypass part, a swing frequency adjustment module (an upper permanent magnet, lower and upper permanent magnets with different polarities, electromagnet structures with the same polarities, a feedback circuit or a flexible sealing sleeve, and a pressure control assembly composed of a gas pump and a gas valve ), and a fixing base. The invention provides a method of making the swing frequency of a flowbypass body resonating with the vortex shedding frequency of the wake in a flow bypass movement, the flow bypass part swings left and right with a semi-flexible support member under the influence of wake vortex shedding, a feedback circuit is used for detecting the incoming flow velocity and the swing frequency of the flow bypass part, the swing frequency of the flow bypass part is adjusted by thefeedback circuit to a resonance region at the incoming flow velocity, and the swing frequency of the flow bypass part is made to resonate with the vortex shedding frequency of the wake flow field. The structure has advantages of strong controllability, no lubrication and maintenance and long service life.