The invention discloses a laser scanning confocal microscope in-situ fatigue crack observing method and belongs to the technical field of fatigue crack detection. The method comprises the steps: preparing a fatigue test sample in a shape of a parallel section with shoulders; performing polishing treatment on the surface of the test sample; installing and exerting 1 to 5 newton force on the test sample; setting the maximum force value as yield strength*stress area*80% and the minimum force value as 0N; setting a loading and unloading velocity as loading or unloading the maximum force N/S per second and a frequency as 0.5Hz; analyzing and storing experimental data and video results after an experiment is finished; choosing typical videos, converting in images, measuring crack length a, finding out corresponding fatigue times N and calculating relationship between the crack length a and the fatigue times N; obtaining a crack spreading rate da/dN. The method has the advantages of simpleness in operation, accuracy in measurement and ability in tracing and researching the relationship between fatigue cracks and microstructures and the relationship between inclusion and orientation in situ.