The invention discloses a unilateral thickness control method of a heavy and medium plate mill. The unilateral thickness control method of the heavy and medium plate mill comprises the step that roller seam values on the two lateral sides of band steel in the width direction are adjusted independently, so that the outlet thickness of the band steel is controlled. The unilateral thickness control method of the heavy and medium plate mill can achieve control over the whole thickness of the band steel and control over the unilateral band steel thickness in the width direction at the same time, not only improves the thickness precision of the two sides of the band steel in the width direction, but also improves the thickness precision of the whole band steel.