In this paper we study theoretically the degradation phenomenon of GaN-based resonant tunneling diode (RTD). The effects of trapping centers on GaN-based RTD are calculated and studied by self-consistently solving the Poisson-Schrödinger aligns when three experimentally obtained deep-level trapping centers are introduced into the AlGaN/GaN/AlGaN quantum well. Results show that the degradations of negative differential resistance (NDR) characteristic in GaN-based RTDs are actually caused by the combined action of the activation energy and the defect density. The deep-level trapping center with high activation energy plays a dominating role in the degradation of NDR characteristics because the probability of ionization is exponentially proportional to the activation energy.