Abstract: The reactive hot pressing method is one of the important methods for preparing Ni Al alloy sheets. For this method, pore defect is an important factor affecting the mechanical properties of Ni Al alloy sheet. Taking Ni and Al foils as raw materials, Ni Al alloy plates were prepared by reaction synthesis method. The evolution law of pore defect in Ni Al alloy sheet under different temperature, time and pressure was studied. The results show that the reaction temperature mainly affects the element diffusion rate and phase composition, and it has little effect on the change of pores. With the increase of reaction time, the pores gradually aggregate, the number of pores decreases, the large pores begin to self-heal, and the average pore size and porosity decrease. Pressure is the most important factor affecting the pores. With the increase of pressure, the number and size of pores show an obvious downward trend. At 10 MPa, the porosity drops below 0.1%, and at 20 MPa, the pores are basically completely eliminated.