A sliding door safety device for heavy construction equipment, comprising: an outer side wall having a doorway formed thereon; an upper rail and a lower rail formed on an upper part and a lower part of the outer side wall, respectively; a sliding door sliding to move to a front or rear side of a cab by an upper roller and a lower roller engaged with the upper rail and the lower rail, respectively, to open or close the doorway; and an upper roller support bracket installed on the inside of the sliding door so that the upper roller is engaged with the upper rail to be rotatable, characterized by first and second door position sensing switches (22, 22') mounted on one side of the outer side wall to sense a door position when the door is in its open position where the door is fully opened or in its closed position where the door is fully closed; a switch driving member for driving the first door position sensing switch, wherein the switch driving member is provided on the upper roller support bracket, a lower roller support bracket is installed on the inside of the sliding door so that the lower roller is engaged with the lower rail to be rotatable, a switch driving member for driving the second door position sensing switch is provided on one side of the lower roller support bracket, if the upper roller and the lower roller move along their respective rails (4, 5) and the sliding door secedes from the fully open position either of the first and second door position sensing switches (22, 22') generates a door secession signal, and if the upper roller and the lower roller move along their respective rails (4, 5) and the sliding door secedes from the fully closed position either of the first and second door position sensing switches (22, 22') generates a door secession signal. an outer side wall having a doorway formed thereon; an upper rail and a lower rail formed on an upper part and a lower part of the outer side wall, respectively; a sliding door sliding to move to a front or rear side of a cab by an upper roller and a lower roller engaged with the upper rail and the lower rail, respectively, to open or close the doorway; and an upper roller support bracket installed on the inside of the sliding door so that the upper roller is engaged with the upper rail to be rotatable, characterized by first and second door position sensing switches (22, 22') mounted on one side of the outer side wall to sense a door position when the door is in its open position where the door is fully opened or in its closed position where the door is fully closed; a switch driving member for driving the first door position sensing switch, wherein the switch driving member is provided on the upper roller support bracket, a lower roller support bracket is installed on the inside of the sliding door so that the lower roller is engaged with the lower rail to be rotatable, a switch driving member for driving the second door position sensing switch is provided on one side of the lower roller support bracket, if the upper roller and the lower roller move along their respective rails (4, 5) and the sliding door secedes from the fully open position either of the first and second door position sensing switches (22, 22') generates a door secession signal, and if the upper roller and the lower roller move along their respective rails (4, 5) and the sliding door secedes from the fully closed position either of the first and second door position sensing switches (22, 22') generates a door secession signal.