In the Huitengxile grassland of the Wulanchabu City, Inner Mongolia, there are hundreds of Maar craters with negative topography. They are numerous, densely distributed and well preserved. Their main features are all negative terrain and no crater rim above the surface rock. Basaltic layer and its fragments as the caprock cover the slope and bottom of the crater and there are small cliffs at the edge of the crater, which also have the composition of the caprock. Geophysical examination results also show that basaltic fragments hidden by soils cover the bottom and slope of the crater. Core drilling revealed at least five rock fracture zones, indicating that the possible magmatic fluid would have cemented these blocks. A comprehensive analysis shows that the magma encountered rich aquifer, produced water vapor, which accumulated energy after encountering barrier layer in the process of rising and led to underground explosions. The collapse of the surface rock began at the center of the crater and resulted in a wedge- shaped collapse around it. The larger Maar crater experienced more explosions than the smaller one. The authors also believe that these Maar craters have the same formation age and origin. Global studies on Maar craters show that the number of ejected debris is determined by amount of the water mixing with the magma. However, there is a lack of research on Maar craters formed by dash and break of steam only when magma encounters water rich layers. This study of the Maars in the Huitengxile basin is an important contribution to the formation of the Maar craters.