The invention provides a gas leakage detection apparatus which comprises a factory compressed gas input port, an air pressure adjusting unit, a gas on-off control unit, a measuring range enlargement selection unit, a gas flow sensor and conditioning circuit, and a clamp unit. The factory compressed gas input port receives compressed gas used for detection, transmits the gas to the air pressure adjusting unit, and after obtaining flowing gas of suitable air pressure, transmits the gas to the gas on-off control unit. The gas on-off control unit is turned on, stable gas is transmitted to the measuring range enlargement selection unit, and the flowing gas is outputted to the clamp unit through the gas flow sensor and conditioning circuit. According to the invention, the measuring range enlargement selection unit is added, through a gas flow measurement apparatus, outputted gas reaches the clamp unit equipped with a device to be measured, a disadvantage that in previous airtight measurement an object to be detected should has certain internal volume is overcome.