This book systematically introduces the moving particle semi-implicit (MPS) method and its improvements in the mechanical engineering. The book is composed of six chapters. The first chapter introduces the classification of mesh and meshless methods. The second chapter carefully describes the derivation of the MPS method. Chapter three reviews the different wall boundary conditions used in the MPS method and emphasizes the polygon wall boundary condition that is an important wall boundary condition widely employed in the industry simulations. Chapter four to six introduces the latest enhancements of the polygon wall boundary conditions in the particle number density and the pressure distributions.The book has clear structure and pays attention to the basic theories and derivations. Many newest research progress of the MPS method are included. The book can be used as the textbook for the postgraduate students in the field of the mechanical engineering, nuclear engineering, naval architecture, ocean engineering and other relative majors. The researchers and engineers can also study the methods in the book to conduct industry simulations.