The invention discloses a device and a method for quickly realizing shortcut biological denitrification through the combination of free ammonia (FA) restraining and a pH value, and belongs to the technical field of sewage biological denitrification. The device mainly comprises a raw water tank, a water inlet pump, a water inlet pipe, an alternate aerobiotic / anoxic reactor, an air compressor, an aeration pipe, an air disperser, a carbon source storage tank, a carbon source feeding pump, a carbon source feeding pipe, a stirrer, a pH determinator, a pH sensor and a water draining pipe. The aerobiotic aeration is implemented in the shortcut nitrification phase; under the condition of sufficient dissolved oxygen, ammonia nitrogen is oxidized to nitrite nitrogen by AOB in sludge, and the pH value is monitored at the same time; when the pH value appears the ammonia valley feature point, the nitrification reaction is finished, and the aeration is stopped immediately; an outer carbon source is added to implement the anoxic stirring, and the pH value is monitored at the same time; when the pH value appears the nitrite knee feature point, the denitrification is finished, and the precipitation and the water drainage are implemented. The pH value accurately controls the nitrification time and denitrification time, so that the shortcut nitrification can be quickly realized and stably maintained, and the bottleneck problem of the shortcut nitrification is solved.