Planetary gear trains (PGTs) are widely used in industrial applications. Failure often occurs under working conditions. Tooth surface spalling is one of the most common defects in a PGT system; it seriously affects the reliability and safety of the mechanical transmission system and may even cause serious incidents. However, research into the faults of planetary gear trains is insufficient, especially with respect to the response characteristics of a PGT in the presence of a spalling defect. The paper designs spalling cases with different localized distributions to demonstrate their influence on the dynamic performance of the planetary gear set. The research provides a theoretical basis for health diagnosis and early fault detection for a PGT system.