To research the influence of overload on combustion stability of solid rocket motor (SRM), numerical simulation and overload test for the $315 motor were done. Through simulating three-dimensional two-phase flow field, the flow-field and particle concentration distribution characteristics of the chamber under overload and non-overload were studied; Linear combustion stability theory was used to calculate detailed motor stability parameters, and combustion stability of the $315 motor was evaluated according to linear stability criterion, the result was well consistent with the test. It demonstrates the change of particle concentration distribution is the key factor causing combustion instability.