Based on DD98M nickel-based superalloy, four kinds of alloys of A1-Ta /(Ta+Ti)=0, A2-Ta /(Ta+Ti)=0.34, A3-Ta /(Ta+Ti)=0.66 and A4-Ta /(Ta+Ti)=1 were prepared by vacuum induction melting, keeping the total amount of γ" phase forming elements of (Ta+Ti) in the alloys unchanged. The as-cast alloys were subjected to solution aging and long-term aging at 1273k. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe (EPMA) were used to study the microstructure of the four alloys after aging. The effects of long-term aging at a high temperature and the change of Ta/(Ta+Ti) on the as-cast microstructure and properties of the alloys were analyzed. The results show that long-term aging causes partial decomposition of γ" phase, promotes element diffusion and intensifies element segregation. With the extension of long-term aging time, the γ" phase coarsens, the hardness decreases, the absolute value of misfit decreases, and the cubic degree of γ" phase decreases. With the increasing proportion of Ta in (Ta+Ti), the absolute value of misfit decreases, the cubic degree of γ" phase decreases, and the hardness increases. Under the long-term aging condition, the segregation of Cr, Mo, W and Ta is intensified, and the segregation of Ti is alleviated. The addition of Ta will squeeze W into the γ phase. After long-term aging, σ phase and MC carbide precipitated at the grain boundaries of A2 and A3 alloys, while no precipitation was observed at the grain boundaries of A1 and A4 alloys, indicating that the synergistic effect of Ta and Ti promotes the precipitation of σ phase and MC carbide. A2 alloys have relatively high γ" phase volume fraction, γ" phase cubed degree, hardness and strength, the smallest γ" phase size and the highest elongation, so A2 alloys have the best comprehensive mechanical properties of the four alloys.