The invention discloses an online detection method for the cooling work state of a hot rolled strip steel work roller, and belongs to the technical field of hot rolling strip steel mill detection methods. The online detection method is used for conducting online detection on the work roller cooling work state of a hot rolled strip steel mill. According to the technical scheme, after the mill shuts down, work roller cooling water supply is not stopped, the work roller cooling water with the work pressure is kept to be continuously sprayed to the static work roller, a thin and uniform reddish brown rust layer is rapidly generated on the roller face of the work roller under the oxidization action of cooling water, but the part directly impacted by the work roller cooling water is hardly provided with the reddish brown oxidation rust layer, the impact mark of the work roller cooling water can be easily distinguished on the reddish brown roller face, and the work state of the work roller cooling water can be judged according to the impact mark. The detection method is simple, convenient to conduct and feasible, all rack work rollers of a production line can be synchronously detected, the detection time is greatly shortened, the problems that an existing detection method is long in time and high in expense are solved, and obvious economic benefits are achieved.