An optical system, a method and an apparatus for diagnosing the optical system are provided. The optical system includes a laser sensor, and a tubular sleeve surrounding the laser sensor and configured to move upwards or downwards relative to the laser sensor to cover or expose the laser sensor. The method includes: driving the tubular sleeve to cover the laser sensor to form an enclosed chamber; filling the enclosed chamber with fluid; controlling at least one of the one or more transmitters to emit an optical signal into the fluid; controlling at least one of the one or more receivers to detect a scattered signal of the optical signal; determining whether the at least one of the one or more transmitters or the at least one of the one or more receivers has a fault based on the scattered signal.