Even though the numerical solution of the unsaturated water flow in soil has been widely applied, the analytical solution cannot be replaced in understanding the intrinsic mechanism of water flow in soil Considering the nonlinear in nature of unsaturated water flow in soil, analytical solutions should be extremely difficult to obtain, especially as the gravity is taken into consideration Thus, so far, there is not any analytical solution available for those issues with both the gravitational effect and the nonlinearity under consideration In this contribution, a coordinate transformation approach is suggested, enabling us to avoid the obstacle of the gravity by removing the item of gravitational effect in the equation Subsequently, the equation for describing unsaturated water flow in the gravitational field bears the same form as the equation neglecting the gravitational effect As a result, for the first time, an analytical solution that can be expressed with the algebra form is obtained The relational theoret ical results, including the new method of coordinate transformation and the analytical solution advanced in this paper, have been verified in experiments.