The invention belongs to the technical field of advanced manufacturing, and discloses a laser additive manufacturing forming method for reinforcing the mechanical property of a pentamode material in situ. The method comprises the following steps of (1) providing a matrix powder material, doping the surface of the matrix powder material with a reinforcing material to obtain a pre-mixed material, and then screening the pre-mixed material to obtain a mixed material; and (2) carrying out printing direction designing and layered slicing processing on a three-dimensional model of pentamode materialstructural parts to be manufactured, and then according to the three-dimensional model, carrying out selective laser melting processing on the mixed material to manufacture the pentamode material structural parts, wherein the matrix powder material and the reinforcing material generate in-situ reaction under the action of laser to generate a new compound, and the new compound is located on the particle boundary of the matrix powder material in a spatial uniform distribution mode. The method has the advantages that the process is simple, the mechanical property of the pentamode material structural parts is improved, and the cost is reduced.