The invention discloses an alarm device for flat tire of trailer of transport vehicles, comprising a direct-current power supply, a start switch, an identifying driver, an integrated circuit, a first mercury tilt sensor, a second mercury tilt sensor, a first resistor, a second resistor, a third resistor, a fourth resistor, a first diode, a second diode, a third diode, a first slide rheostat, a second slide rheostat, a first audion, a second audion, a third audion, a first light emitting diode, a second light emitting diode and a loudspeaker. In application, the two mercury tilt sensors are respectively installed at two sides of the trailer of the transport vehicle; if occurring in flat tires, the trailer will incline; the two ends of the mercury tilt sensor at the flat-tire side are conducted; and finally the loudspeaker is led to alarm. According to the invention, flat tires of the trailer of the transport vehicle can be automatically detected; gas leakage alarm can be automatically sent to a driver; and then the driver can process failure timely and ensures driving safety.