An alternating pressure melt impregnation device and a melt impregnation method, including having a resin melt squirted from each resin melt runner on an upper die and a lower die of a melt injection area, and thus the squirted resin melt is enable to be squirted directly on an upper surface and a lower surface of a continuous fiber bundle which is entering into an impregnation chamber. Impregnation and infiltration for both surfaces of the continuous fiber bundle are primarily completed by a squirted pressure. The resin melt inside the impregnation chamber flows to a decompression chambers at both sides of the impregnation chamber. When the resin melt flows to a throttle plate, a re-impregnation for the continuous fiber bundle is realized. Then the pressure is decreased and a section of the resin melt is enlarged and a radial flow is generated due to the Barus effect.