The invention discloses a method for observing three-dimensional shapes of nonmetallic inclusions in steel in situ and belongs to the technical field of metal detection and analysis. The method comprises the following steps: preparing a to-be-observed and detected test sample; adding an erosion solution into an electrolyzing device and immersing the observing and detecting surface of the test sample in the erosion solution; setting the voltage and the erosion time for electrolytic attack of the observing and detecting surface of the test sample, wherein a steel matrix on the shallow layer of the observing and detecting surface of the test sample is dissolved and the nonmetallic inclusions are exposed; and after erosion, cleaning and blowing the test sample, and observing the three-dimensional shapes of nonmetallic inclusions through a scanning electron microscope. According to the method, the true three-dimensional shapes of the nonmetallic inclusions in the test sample are not damaged, information such as positions and distribution of the nonmetallic inclusions in the test sample are not changed, the shapes of the nonmetallic inclusions in steel and generation, development and evolution processes of steps of full course of the inclusions can be correctly known, and guiding action is provided for removing or denaturating the inclusions and improving the cleanliness of molten steel.