The invention belongs to a composite material forming technology and relates to a forming method capable of improving porosity of a honeycomb sandwich structure composite material. The forming method of the honeycomb sandwich structure composite material comprises cementing treated honeycomb and skin to form a honeycomb sandwich structure; solidifying the honeycomb sandwich structure: during solidification, vacuumizing firstly, pressurizing step by step, closing vacuum and opening to atmosphere, increasing the temperature at a specific rate to perform heat treatment to completely discharge micromolecular gas, continuously heating and solidifying, and then cooling and relieving pressure. By the method provided by the invention, resin of skin completely flows, the micromolecular gas is effectively discharged, the porosity of the honeycomb sandwich structure composite material is greatly improved, the mechanical property of parts can be guaranteed, the nondestructive detection passing rate of the obtained products is high, the qualification rate of the products can reach above 95 percent, and the forming quality requirement of the products can be met well.