The invention discloses an alloy melt hydrogen removal device and an online hydrogen removal method thereof. The device comprises a purifying tube which comprises an alloy melt inflow end and an alloy melt outflow end, Ti-Mo alloy particles are evenly placed into the purifying tube, and the alloy melt outflow end is provided with a first sealing plate used for filtering the Ti-Mo alloy particles. The alloy melt hydrogen removal device is simple in structure, convenient to install and replace, low in hydrogen removal cost, good in hydrogen removal effect and easy to apply to industrial production. Besides, the online hydrogen removal method based on the alloy melt hydrogen removal device is few in process procedure, equipment is convenient to install and replace, and the hydrogen removal effect is good. A magnesium alloy melt prepared by means of the device and the method can remarkably reduce the content of hydrogen in the melt, metallurgical quality of castings is improved, and the device and the method have good application prospect in industrial production.