Magnesium-based water ramjets with two water intakes are known for their better performances than those with only one intake. This article studies the design of the combustion chamber length and water intake distance on this type of water ramjets. Theoretical anlysis and experimental temperature data indicate that magnesium enters the combustion chamber as molten droplets, and the burning process of the droplets is controlled by vapor-phase diffusion. A simplified D^3/2 law is used to calculate the droplet burning time. A method to design the combustion chamber length and water intake distance is developed on the basis of magnesium burning time and water evaporization time. The structure of a water ramjet prototype is improved with this method, and comparison tests are conducted for the two structures in a ground direct-connect testing system. An enhanced engine performance is obtained with the improved structure in the tests, which also testify the validity of the design method.