The invention discloses a method for controlling the convexity of a heavy and medium plate rolling mill working roll. An upper working roll inlet cooling water collecting pipe and an upper working roll outlet cooling water collecting pipe are separated into three cavities; at least one nozzle is formed in each cavity; the nozzles of the three cavities of the upper working roll inlet cooling water collecting pipe respectively face the two ends and the middle section of an upper working roll; the nozzles of the three cavities of the upper working roll outlet cooling water collecting pipe respectively face the two ends and the middle section of the upper working roll; at least one water inlet is formed in each cavity and is respectively connected with a water supply system; a third flow rate adjusting valve and a fourth flow rate adjusting valve are respectively arranged on a communicating pipeline; the third flow rate adjusting valve controls the flow rate of cooling water in the cavity in the middle of the upper working roll inlet cooling water collecting pipe to be not greater than the flow rate of cooling water in the cavities at the two ends of the upper working roll inlet cooling water collecting pipe; the fourth flow rate adjusting valve controls the flow rate of cooling water in the cavity in the middle of the upper working roll outlet cooling water collecting pipe to be not greater than the flow rate of cooling water in the cavities at the two ends of the upper working roll outlet cooling water collecting pipe.