The invention provides a plate shape control method of an online-quenched high-strength steel plate. The high-strength steel plate is formed by continuously casting blanks through the plate shape control method. The plate shape control method comprises the following steps of 1, heating, 2, descaling, 3, rolling, 4, online quenching, 5, retarded cooling and 6, tempering. The plate shape control method is applicable to a medium-thickness plate production line without arrangement of a pre-straightening machine, through a plate shape control technology, the plate shape quality of the online-quenched high-strength steel plate is improved, after tempering, the plate shape unevenness is controlled to be 7mm/2m or below, hardening and tempering for remedy and size changing are avoided, the production cost is reduced, and the yield is increased.