In this work, the effects of hot isostatic pressing and heat treatment (solution and double aging) on the high-temperature tensile properties of a nickel-based polycrystalline superalloy K417G were investigated. The experimental results indicate that following the hot isostatic pressing and heat treatment, the porosity of the alloy decreases from 0.072% (in as-cast state) to 0.043%. The volume fraction of γ' phase increases from 43.28% to 56.54%, and the shape tends to be more cubic. The plasticity of the superalloy at 900 °C increases from 5.1% in as-cast state to 9.8% followed HIP and HT, but the tensile strength remains unchanged. Large size micropores are present in the as-cast sample and cracks sprout and expand at the micropores. After hot isostatic pressing and heat treatment, micropores are effectively eliminated, preventing them from becoming sources of cracks. This delay in the emergence of cracks results in enhanced plasticity. In addition, dislocations in the specimens after hot isostatic pressing and heat treatment bypasses the γ' phase through the Orowan mechanism, leading to a further increase in plasticity.