In order to study the development of moisture insider concrete slabs, a one dimension water movement model for early age concrete slabs subjected to external drying was established. Adomian decomposition method (ADM) as an approximate analytical solution to initial boundary value problem (IBVP) of water movement equation for moisture distributions in early age concrete slabs with arbitrary boundary conditions was derived. Comparison between numerical results by Crank Nicolson finite difference scheme and experimental results suggests that the proposed ADM based approximate analytical solution approach converges very fast and is effective for solving nonlinear diffusion equation with arbitrary boundary and initial conditions as well as any diffusivity coefficient function for external drying of concrete slabs.