With the rapid progress and development of technique and equipment in the field of aerospace and weapon protection, it is of great significance to accelerate the development and research of new metal composite materials with lightweight and high-strength properties. In this research, the multiscale numerical calculation method was used to explore the variation law of interface characteristic parameters as well as atomic-scale diffusion behavior and characteristics of the interface of explosive welded multilayer metal composite materials. The results show that with the passage of time, the dynamic collision angle increases slightly in the initial stage and remains stable in the middle stage. The joining interface shows obvious waveform structure characteristics. The pressure distributed at the joining interface of the multilayer composite sheet is significantly higher than that in other regions of the sheet, and the pressure of the bonding interfaces decreases gradually from top to bottom. The effective plastic strain at the bottom interface is slightly higher than that at the other three interfaces. Under microscale collisions at different speeds, obvious atomic diffusion occurs at all three solder joints. As the impact speed decreases, the thickness of the atomic diffusion layer at the interface also decreases. The thickness of the three diffusion layers ranges from 1.12 μm to 1.58 μm, 1.8 μm to 2.55 μm and 1.22 μm to 1.73 μm.