The utility model discloses an automatic automobile-mounted alcohol detecting device which comprises a control box, a gas sensor, a digital-to-analog conversion module, a control unit, a normally open relay and a buzzer, wherein the gas sensor is installed on a lower surface of a sun shield above a head of a driver, and other components are packaged in the control box which is installed below a center console in a cab; a signal output end of the gas sensor is connected with an input end of the digital-to-analog conversion module, and an output end of the digital-to-analog conversion module is connected with an input end of the control unit; and the normally open relay and the buzzer are respectively connected with an output port of the control unit, and a control end of the normally open relay is connected with a starting circuit of an automobile engine. By using the device, gas exhaled by each driver can be stably and effectively measured, if the breathing alcohol content of the driver exceeds the national limit value, an automobile can be prevented from being started, and the drunk driver is technically prohibited from driving.