The invention provides an automatic pressure-adding borehole sealing device and method. The device comprises a gas extraction pipe, two expandable pressure-resistant bags for containing hole sealing slurry are sleeved on the gas extraction pipe at intervals, the two bags and the space between the two bags are respectively communicated with a grouting device through grouting pipes, a pressure meter is mounted on each grouting pipe, and a grouting control device obtains the pressure of the slurry in the grouting pipes by acquiring data of the pressure meters and controls the grouting device to respectively inject the hole sealing slurry into the grouting pipes. When primary sealing of a borehole affected by mining cannot meet extraction requirements, the grouting control device controls the grouting device to respectively inject the slurry to the two bags and the space between the two bags, secondary or subsequent grouting pressure-adding hole sealing is performed, extraction effects are enhanced, the problem that an existing grouting device and method cannot timely seal space formed by redevelopment of cracks on the periphery of the borehole after deformation to fail a borehole plugging process is solved, and the device is high in practicability and simple to operate.