The K439B alloy is a novel equiaxed superalloy and is used for producing hot section components that need to resist high temperatures in aero engines and gas turbines as its temperature capacity exceeds 800^oC. In this study, the evolution of the microstructure and mechanical properties of K439B equiaxed superalloy after being subjected to long-term aging at 800^oC for 3000 h was examined. The predominant deformation mechanisms affecting room-temperature tensile and stress rupture properties at 815^oC and under 379 MPa stress following different aging durations for the K439B alloy were investigated.Results indicate that for heat-treated alloy, the morphology of the γ' phase is spherical, MC carbide is generated in the interdendritic region and grain boundaries, while M₂₃C₆ carbide is in the grain boundaries. During long-term aging at 800^oC, γ′ precipitates conform to the Ostwald ripening mechanism for growth and tend to take a cubic form; the coarsening rate of the γ′ phase is calculated to be 71.7 nm³/h; Additionally, the MC carbide deteriorates while the content of M₂₃C₆ carbide gradually increases. After long-term aging for 3000 h, the precipitated grain boundary phase comprises MC carbide, γ′ phase, and M₂₃C₆ carbide; the orientation relationship between γ′ phase and M₂₃C₆ carbide can be described as [111] _γ' //[111] _M{}_{{}_{\mathrm{23}}}_C{}_{{}_{\mathrm{6}}} and (\mathrm{2}\overline{\mathrm{2}}\mathrm{0}) _γ′ //(\mathrm{2}\overline{\mathrm{2}}\mathrm{0}) _M{}_{{}_{\mathrm{23}}}_C{}_{{}_{\mathrm{6}}}. The heat-treated alloy demonstrates room-temperature tensile and yield str-engths of 1159.0 MPa and 911.5 MPa, respectively. Meanwhile, the stress rupture life at 815^oC and under 379 MPa stress is 150.4 h. As the size of γ′ precipitates increases, the dominant deformation mechanism shifts from dislocation slipping in the matrix to dislocation cutting through the γ′ phase after long-term aging, resulting in superior stacking faults appeared in the γ′ phase. Consequently, the room-temperature tensile strength and stress rupture life show reduction at 815^oC and under 379 MPa stress.