A sliding armrest device for a vehicle may include a fixed armrest having a first oblique mounting surface formed on a top surface thereof and inclining along a width direction of a vehicle body, a sliding armrest having a second oblique mounting surface formed on an undersurface thereof and vertically matching the first oblique mounting surface, and a sliding rail device disposed between the first oblique mounting surface of the fixed armrest and the second oblique mounting surface of the sliding armrest and providing a forward and backward sliding force for the sliding armrest.