An apparatus and a method for detecting a leakage of heavy water in a nuclear reactor system includes: a diode laser that injects a generated laser beam to a test sample placed in a light absorption cell; a vacuum pump adjusts a degree of vacuum in the light absorption cell; a test sample introduction unit gathers an air test sample from a location with a high possibility of a leakage of heavy water or a light water test sample from a secondary side of a steam generator and transfers the sample to the light absorption cell; an optical detector detects the laser beam which has passed through the light absorption cell; and a microprocessor controls the operation of the diode laser, the vacuum pump, the test sample introduction unit, and the optical detector, receives a detect signal from the optical detector, and analyzes an absorption spectrum signal.