The effects of Hf addition on the microstructure, solidification behavior and porosity of a nickel-based high-boron cast superalloy were studied. The results of microstructure analysis, differential thermal analysis (DSC) and isothermal solidification quenching test showed that Hf segregated in interdendritic regions during the solidification process of the alloy, and precipitated from the residual liquid phase in the form of Ni5Hf phase at the end of the solidification process. The addition of Hf reduced the precipitation temperature of liquid/solidus , γ/γ′ eutectic phase and boride, delayed the solidification process of the alloy, widened the solidification temperature range from 166.3 ℃ to 200.5 ℃, and significantly increased the content of γ/γ′ eutectic phases. A thin-walled tube cast was independently designed by ourselves. Penetration test results showed that the addition of Hf significantly reduced the porosity of the thin-walled tube, and the tendency of porosity forming of the alloy was significantly reduced.