In view the problem that the instantaneous dust content in air is high and pollution is serious at the hydraulic support caving mouth in the mining face, the research of negative pressure dedusting mechanism is put forward. Through the research of the dust subsidence rule in the caving mouth, discovered that the key of the falling dust should be put on the small dust particles below 30um on the breathing level (which is 1.5~2 meters from the ground). According to the dust subsidence rule, the dedusting efficiency simulation research of the negative pressure dedusting device is conducted, which has provided the theory and the design basis to the negative pressure dedusting device's design.