A sealing device for a collet chuck has a tensioning nut, a sealing washer, a main sealing ring and a pressure space. The sealing washer is mounted in the tensioning nut and has a main ring groove, an abutting surface, a liquid-inlet surface, and a liquid inlet. The liquid inlet is formed between an imaginary surface extending from the abutting surface and the liquid-inlet surface. The main sealing ring is mounted and positioned in the main ring groove. The main sealing ring protrudes out of and abuts the main ring groove. The pressure space is formed between a rear side of the main sealing ring and the main ring groove. Therefore, a coolant poured into the sealing device can pressure the main sealing ring to deform the main sealing ring, such that the main sealing ring can tightly abut a tool shank.