A device for displaying fuel efficiency of construction machinery, comprising: 1.1 a plurality of sensors configured to sense at least one of a displacement of a joystick, a hydraulic pressure, and a pressure of a pump, and transmit sensed information; 1.2 an alternator configured to output a voltage representing rotary force of an engine; 1.3 an engine control unit configured to calculate engine revolutions per minute, instant fuel efficiency, and torque data; 1.4 a display unit configured to include an eco symbol displaying an instant amount of operation load, an eco gauge displaying average fuel efficiency, and a fuel efficiency system displaying instant fuel efficiency or average fuel efficiency; 1.5 an equipment control unit configured to determine a current state of equipment based on the sensed information received from the plurality of sensors and the engine RPM received from the engine control unit, change and display the eco symbol of the display unit according to the current state of the equipment, calculate average fuel efficiency by using instant fuel efficiency and torque data calculated by the engine control unit, and display the calculated average fuel efficiency or instant fuel efficiency through the eco gauge and the fuel efficiency system of the display unit; 1.6 a memory unit configured to store the instant fuel efficiency and the average fuel efficiency; 1.71 wherein when a pressure of the pump is equal to or larger than a predetermined pressure of a relief pressure, or the engine RPM is equal to or smaller than a minimum RPM for controlling the pump, the equipment control unit displays the eco symbol of the display unit with red; and 1.72 when the engine RPM is "0", the equipment control unit displays the eco symbol of the display unit with gray. 1.1 a plurality of sensors configured to sense at least one of a displacement of a joystick, a hydraulic pressure, and a pressure of a pump, and transmit sensed information; 1.2 an alternator configured to output a voltage representing rotary force of an engine; 1.3 an engine control unit configured to calculate engine revolutions per minute, instant fuel efficiency, and torque data; 1.4 a display unit configured to include an eco symbol displaying an instant amount of operation load, an eco gauge displaying average fuel efficiency, and a fuel efficiency system displaying instant fuel efficiency or average fuel efficiency; 1.5 an equipment control unit configured to determine a current state of equipment based on the sensed information received from the plurality of sensors and the engine RPM received from the engine control unit, change and display the eco symbol of the display unit according to the current state of the equipment, calculate average fuel efficiency by using instant fuel efficiency and torque data calculated by the engine control unit, and display the calculated average fuel efficiency or instant fuel efficiency through the eco gauge and the fuel efficiency system of the display unit; 1.6 a memory unit configured to store the instant fuel efficiency and the average fuel efficiency; 1.71 wherein when a pressure of the pump is equal to or larger than a predetermined pressure of a relief pressure, or the engine RPM is equal to or smaller than a minimum RPM for controlling the pump, the equipment control unit displays the eco symbol of the display unit with red; and 1.72 when the engine RPM is "0", the equipment control unit displays the eco symbol of the display unit with gray.