This book is divided into 9 chapters. Chapter 1 is the introduction. Chapters 2 to 4 study the internal flow field and spatter particles' tracks in the metal additive manufacturing printer,and design an innovative fluid cover to optimize the internal flow field of the printer and employ a negative pressure tube to remove spatter particles. Chapters 5 to 9 mainly study the characteristics of the molten pool in the metal additive manufacturing process. Chapter 5 studies the dynamic characteristics of the molten pool. Chapter 6 studies the effect of the external magnetic field on the molten pool and the solidification process in the additive manufacturing process.Chapter 7 and Chapter 8 study the evolution processes of single pore defects and multi-pores defects inside the workpiece in the metal additive manufacturing process.Chapter 9 studies the laser polishing processes of metal additive manufactured workpieces to control the surface roughness.