Turbine blades play a decisive role in the performance of turbomachinery. With a disc-mill cutter, the whirling milling process has been proposed as an efficient, economical, and innovative approach to machining blades, during which cutting force has a significant influence on the accuracy of the blade. In this paper, a three-dimensional finite element model is established to simulate the tangential, radial, and axial milling forces. Based on the principle of equal instantaneous cutting area of the chip, the workpiece, cutting tool, and chip with complex geometry are simplified into a straight-line model. According to the simulation results, the relationship between instantaneous cutting area and milling force is obtained. The tangential force is the largest, followed by the radial force, and the axial force is the smallest. Finally, the simulation results are verified by predicted forces within acceptable proximity, which demonstrate that the established model based on instantaneous cutting area is feasible and effective in simulating the tangential, radial, and axial forces in the actual whirling milling process.