Abstract: The paper analyzes the reasons of cracks occurred twice in the forging of 14Cr17Ni2 Martensitic Stainless Steel forging used for the reactor internals. Through the calculation of phase diagram, and the experimental research of the influence on the content and pattern of δ ferrite in different heating temperature, different holding time and forging temperature, the paper concludes that the higher the heating temperature, the longer the holding time, the lower the forging temperature, the more easy the cracks appearance. Results show that no crack appears in the forging after the improvement of the heating temperature, holding time and finish-forging temperature.