Abstract: This paper utilizes the three-dimensional toroidal nonlinear magnetohydrodynamic code CLT to analyze the excitation process of the m/n=2/1 neoclassical tearing mode (NTM) seed magnetic island by sawtooth oscillation, and analyzes the long-term effects of sawtooth oscillation on the m/n=2/1 magnetic island with or without bootstrap current. The results indicate that: (1) Before the occurrence of the sawtooth crash, the m/n=2/1 magnetic perturbation component has already started to grow. When the sawtooth crash occurs, the growth rate of the m/n=2/1 magnetic island width increases and the duration is longer, due to the radial outward movement of the m/n=2/1 magnetic perturbation caused by the sawtooth crash; (2) During the sawtooth oscillation, the m/n=2/1 magnetic island can grow regardless of the presence of bootstrap current. When there is a bootstrap current, the m/n=2/1 magnetic island is primarily driven by the neoclassical effect; when there is no bootstrap current, the growth of the m/n=2/1 magnetic island is driven jointly by the sawtooth and viscosity effects.