The definition principle of ratio of width to height of a lock wall in the design of monolithic lock chamber structure is studied, and the result shows that the range of the ratio in the current lock criterion is not suitable for high water-head locks. Based on the material mechanics theory, a formula describing minimum width-height ratio of a lock wall for all kinds of concrete strength is presented under different design water-heads, and this formula can conquer the conventional criterion s shortcoming caused by experience. By some FEM numerical examples, the results indicate that the formula could satisfy the strength and crack restricting requirements in the global-shaped lock structure, while it has the advantages of simple form and convenience for popularization. So, it is a good supplement for current lock design criterion.