Abstract: An approximate analytical method for determining the critical separation distance of adjacent buildings based on seismic pounding fragility is proposed. Pounding events are expressed in the form of a single-barrier first-passage reliability problem according to the performance-based seismic design theory. With the random vibration theory, the conditional failure probabilities of a structural system under various seismic intensities are obtained. Combined with the seismic pounding fragility of adjacent buildings, the calculation of critical separation distance is descried as an inverse reliability problem. The critical separation distance corresponding to a targeted pounding probability during the design life is derived. The applicability and validity of the above theory are verified by the Monte Carlo numerical simulation method.