A desire to further comprehend the hydrodynamic characteristics of three-dimensional undulating fin propulsion is what motivates the work. First, a high-resolution numerical technique based on the constraint immersed boundary method is utilized to simulate the fluid-fin system. The simulation results reveal fundamental variation laws between the hydrodynamic performance of the undulating fin and kinematic parameters. According to the simulation data, an in-depth analysis of the scaling law is conducted. A key contribution of this work is to build the force scaling formula and extend the law to complicated cases involving different incoming flow velocities. The important application of the force scaling law is that it can be used to estimate the self-propelled speed and wave efficiency of the undulating fin in different kinematic conditions. The results show that the wave efficiency exhibits a monotonically bounded increasing trend as the amplitude grows, is basically independent of the frequency, and decreases monotonically with the increasing wavelength. Finally, the work discusses the evolution of vortex structures in undulating fin propulsion. The analysis indicates that the streamwise central jet formed throughout the fin body is the primary reason for thrust generation in undulating fin propulsion. Furthermore, the basic dynamic mechanisms of two types of vortex rings, related to the formation of the central jet, are investigated in the work. The results further reveal the macro-interaction mechanism between the undulating fin and fluid flow. The findings could make a contribution to explaining some biological phenomena and developing bionic engineering.