The sheet-bulk metal forming (SBMF) technology provides a new processing method for gears. This manufacturing technology is greatly affected by process parameters, deformation conditions and material properties. Defects such as holes, tearing and insufficient filling of materials are easy to occur during forming process. In order to understand the mechanism of this process, finite element modelling and simulations are carried out on the sheet-bulk forming of gears. Equivalent stress and material flow are analyzed by combining point tracking method. The mechanism of incremental sheet-bulk forming of gears technology is clarified. The phenomenon of insufficient filling of initial teeth caused by restricting material flow is emphatically explained.