The utility model provides a diesel oil anti-theft and alarm device of a square oil tank of an engineering truck. The oil tank comprises an oil tank body, wherein an oil inlet and a floating oil portare formed in the upper end of the oil tank body; oil slick is mounted in the oil slick opening; a sleeve is arranged in the oil tank body, a floating plate is arranged on the outer side of the sleeve, a sensing chamber and a fence are arranged on the sleeve, a magnetoelectric vibration sensor and a stm32 single chip microcomputer are arranged in the sensing chamber, two alarm loudspeakers are connected to the stm32 single chip microcomputer, one alarm loudspeaker is installed in a vehicle, and the other alarm loudspeaker is installed outside the vehicle. According to the utility model, secondary burglary prevention can be realized, the detection of the vibration frequency of the oil tank is realized through the magnetoelectric vibration sensor and the stm32 single-chip microcomputer processing system, and when the vibration reaches a threshold value, the device gives an alarm to realize primary burglary prevention; the floating plate and the oil surface are attached to move up and down, so that oil is isolated from the outside, secondary burglary prevention is achieved, and conversion from prevention to burglary prevention is achieved.