An oil tank floating roof device, includes a plurality of connecting bars and a plurality of floating units, which are respectively secured to the plurality of connecting bars by means a plurality of longitudinal and crosswise holding spaces collectively encircling the floating units. A plurality of locking members are fitted on the peripheries of the floating units, and two sides of the connecting bars respectively form at least one joining groove. The joining grooves respectively enable each of the floating units to longitudinally or crosswisely insert therein. The top portions of the connecting bars form at least one pressure resistant reinforcing plate, and the pressure resistant reinforcing plate is fitted with a plurality of locking portions. Bolts are used to respectively correspondingly pass through the locking portions and bolt into the locking members on the longitudinal or crosswise sides of the base of the floating units.