In order to understand the formation mechanism of impact craters in the complex environment of planetary surfaces, low-velocity impact processes were simulated in a laboratory, and the effects of particle size and moisture content on the dynamic behavior of impact craters were systematically investigated. By adjusting the moisture content, the high-speed camera and particle image velocimetry (PIV) technology were employed to capture the flow characteristics of particles during the impact process. The results show that there are three typical formation modes of impact craters in the wet granular system: the diffusion collapse mode, the accumulative collapse mode, and the no collapse mode. The formation mode is jointly determined by the particle size and moisture content. Through the analysis of the phase diagram of impact crater formation modes, the principle of phase transformation in the phase diagram was discussed in detail. Further analysis of the formation law of impact crater scale reveals that the diameter of the impact crater decreases and tends to stabilize with the increase of moisture content. Under the same moisture content, the larger the particle size of the wet granular material, the larger the diameter of the impact crater. Finally, a power-law relationship for the unified description of impact scale characteristics was derived. The research results can provide experimental basis and theoretical models for understanding the geological evolution of planets.