The invention provides a method for directly obtaining martensite die steel through a laser 3D printing technology. According to the method, a defined amount of Mn, Ni, Cr and other alloy elements are added to iron powder, and within a wide temperature range, martensite transformation is facilitated. The influence rules of laser linear energy density eta (the ratio between laser power and scanning speed) on the density, microscopic structure and mechanical property of an SLM forming part are analyzed, parameters of the laser 3D printing technology are optimized, and a mold of a complex structure in any shape is obtained. Moreover, the forming mold has high hardness, abrasion resistance and good impact toughness, and is better in performance than a mold manufactured in a traditional method. Accordingly, the method is particularly suitable for rapid forming of molds of a complex structure.