The invention discloses an on-line detection device and method for gas-liquid transfer method escaping ammonia. The online detection device comprises a fluid flowmeter, an absorption liquid supply system, a smoke dust filter, a liquid membrane heat absorption tube, a gas-liquid first-stage separator, a flow-through tank, a measurement control system, a water quality ammonia nitrogen sensor used for detecting the concentration of NH4<+> contained in absorption liquid in the flow-through tank, and a liquid density sensor used for detecting density of liquid in the flow-through tank. The on-line detection device and method can accurately detect the concentration of escaping ammonia online.