The invention discloses an alloy liquid extrusion casting forming device which comprises a hammerhead (1), a static die (2), a movable die (3) and a pouring slot (4). The device adopts the technical scheme that a declined and downward funnel-shaped pouring slot occupied by both the movable die and the static die arranged respectively by half is arranged on the parting plane on one side of a vertical die casting mould; a cylinder chamber type feed liquid chamber and the piston type hammerhead with a sealing knife is arranged on the static die; the hammerhead is controlled to be pushed up, and simultaneously the sealing knife seals the pouring slot. Firstly, feed liquid is filled into the a die cavity at high speed with low pressure, then the semisolid feed filled into the die cavity is extruded at low speed with high pressure, and the semisolid feed is gradually condensed to be formed under high-pressure low-speed extrusion. The alloy liquid extrusion casting forming device and the process method thereof integrate full casting functions of low-pressure filling and high-pressure extrusion into a whole, ensure that forming of alloy casting achieves the purposes of no gas entrapment, no cast seam, no mis-run, no forging crack, no fash, compact texture, fine crystal grain, high mechanical property, capability of performing thermal treatment and sealing, high dimensional accuracy, energy saving and low cost.