The impact characteristics of a single shot are the fundamental indicators for studying the strength and coverage of mixed jet surface shot peening. The characteristics of the shot and the target material both have an impact on the crater characteristics after impact. This article uses 18CrNiMo7–6 low-carbon alloy steel after different heat treatments as the target material to study the crater characteristics under different shot impact conditions. Starting from the mechanism of crater formation, a single shot impact crater size model was established, and through experimental research, the error of high hardness target crater was corrected, reducing the prediction error of crater depth to less than 30%. On this basis, the laws of crater related characteristic parameters (depth to diameter ratio, proportion of crater diameter and crater depth, and ridge height) were studied. It was found that the hardness of shot particles and target materials had the most significant impact on the depth to diameter ratio and proportion of crater diameter and crater depth, while the ridge like structure around the crater only existed in low hardness target materials. The influence order of each parameter on ridge height is shot diameter, shot hardness, and shot velocity, respectively.