The invention relates to an oil dispersant underwater injection effect assessment method, and belongs to the technical field of marine environment simulation test. The oil dispersant underwater injection effect assessment method comprises the following steps: 1, experiment crude oil is injected to water from the bottom of an underwater overflow oil simulation test apparatus; 2, an oil dispersant is injected in a direction vertical to the injection direction of the crude oil, and is mixed with the crude oil; 3, information of oil drops dispersed by the oil dispersant in the water is acquired through adopting an industrial camera with a microlens; and 4, the change of the particle size of the oil drops is analyzed to evaluate the use effect of the oil dispersant. The method has the advantages of convenience in operation, fastness, high efficiency, accurate assessment of the underwater injection effect of the oil dispersant, direct and effective result, and wide application range.