The utility model provides engineering machinery as well as a device and system for detecting stretching and contracting states of a horizontal supporting leg of the engineering machinery. The device for detecting the stretching and contracting states of the horizontal supporting leg comprises an induction metal block and a proximity switch, wherein the induction metal block is arranged on a movable supporting leg of the horizontal supporting leg; and the proximity switch is arranged on a fixed supporting leg of the horizontal supporting leg and is used for being close to and matched with the induction metal block, so as to trigger a continuity signal to indicate the stretching and contracting states of the horizontal supporting leg. According to the device provided by the utility model, an indication signal of the stretching and contracting states of the horizontal supporting leg is automatically triggered by adopting a detecting device, so that the stretching and contracting states of the horizontal supporting leg can be visually fed back to an operation machinist, the problems that time is wasted and operation is cumbersome as the existing mechanical bolt manner is adopted are solved, and further the effect of improving system efficiency and reliability is achieved.