We numerically investigate the optical bistability from a two-dimensional photonic crystal L6 nanocavity dimer array structure configured under the Su-Schrieffer-Heeger model. The localized electric field in the topological edge state is highly enhanced, which gives rise to strong nonlinear phenomena such as optical bistability. In comparison, a topologically trivial nanocavity is also designed and its field strength distribution and optical bistable response are also simulated. In order to test the robustness, three types of defects and interferences are introduced in both the topologically non-trivial and trivial cavities. Benefiting from the topological feature, the proposed topological cavity exhibits superior optical bistable performance with low threshold power and high switching contrast compared to that in the trivial cavity. Our work suggests what we believe to be a novel avenue toward the insertion of optical bistable devices with high robustness into future photonic integrated circuits and photonic neural networks.