Abstract: Using the finite element method, the influence of the state of charge (SOC), internal defects (such as bubbles, lithium plating, and unwetted), and ultrasonic transmission characteristics on lithium-ion batteries was investigated. First, a multi-layer porous structure of the lithium-ion battery was established using Voronoi polygons. Afterwards, the ultrasonic transmission characteristics of the battery under different SOCs were extracted by changing the mechanical parameters (Young's modulus and density) of the anode and cathode. The simulation results indicate a linear increase in the amplitude of fast P-waves and slow P-waves with increasing SOC, as well as a linear decrease in the time of flight of slow P-waves. Second, various types of internal defects within the lithium-ion battery were simulated. By comparing the ultrasonic transmission characteristics of normal and defective batteries, it is found that a bubble defect at the bottom of the battery caused a significant attenuation of the transmitted signal's acoustic intensity amplitude, which increased with increasing bubble thickness. The position of the bubble also caused regular changes in the acoustic intensity amplitude of the transmission signal. Lithium plating inside the battery resulted in a gradual decrease in the acoustic intensity amplitude and time of flight of the transmission signal as the thickness of lithium precipitation increased. In the presence of unwetted regions within the battery, the simulation model degrades to a single-phase porous medium with only one frequency component in the frequency domain, leading to attenuation of the acoustic intensity amplitude. This research successfully employs the finite element method to simulate the SOC and internal defects of lithium-ion batteries. The simulation results of the slow P-wave velocity are in good agreement with the theoretical results.