An accurate judgment method on tooth undercutting of noncircular gear has been proposed. According to the numerical computation model of shaping, the tooth profile in normal isometric family curves of noncircular gear pitch line and conjugate point in shaper were calculated at first. Then, the theory of tooth undercutting in noncircular gear shaping was analysed by using the essential theory of tooth mesh. Finally, through synthesis of numerical computation model of shaping and theory of tooth undercutting, the accurate judgment method on tooth undercutting of noncircular gear has been obtained, and the numerical estimate method of tooth undercutting has been founded. So, the default of tooth undercutting could be found out in design process of noncircular gear.