Casting experiments and macro-micro numerical simulations were conducted to examine the microstructure characteristics of K439B nickel-based superalloy casting with varying cross-sections during the gravity investment casting process. Firstly, microstructure analysis was conducted on the casting using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Subsequently, calculation of the phase diagram and differential scanning calorimetry (DSC) tests were conducted to determine the macro-micro simulation parameters of the K439B alloy, and the cellular automaton finite element (CAFE) method was employed to develop macro-micro modeling of K439B nickel-based superalloy casting with varying cross-sections. The experimental results revealed that the ratio of the average grain area increased from the edge to the center of the sections as the ratio of the cross-sectional area increased. The simulation results indicated that the average grain area increased from 0.885 to 0.956 mm² as the ratio of the cross-sections increased from 6？1 to 12？1. The experiment and simulation results showed that the grain size became more heterogeneous and the grain shape became more irregular with an increase in the ratio of the cross-sectional area of the casting. CAFE modeling was an effective method to simulate the microstructure evolution of the K439B alloy and ensure the accuracy of the simulation.