This book studies the boundary value problem of three-interval composite second-order differential equation. It obtains the method for solving the class of boundary value problems--Similar Construction Method. After that, the method is applied and extended.The research of this book will further complement and improve the similarity structure theory of boundary value problem of differential equation and widen the application range of the theory. It has a good application prospect in the study of the percolation mechanics of oil and gas reservoirs.