The outdoor design conditions should not only regularly update, but also reflect the effects of climate change. In this study, a new method of calculating the optimal period of record (POR) for air-conditioning outdoor design conditions was put forward. Firstly, the M-K test was used to prove the significantly change of outdoor temperatures in air-conditioning season. Secondly, the generalized Pareto distribution (GPD) model was adopted to fit the distribution of the high temperature and the model was verified by the P-P and Q-Q plot. Empirical distribution functions with different PORs were calculated by defining the meteorological impact factor. Finally, the optimal POR was determined by comparing the empirical distribution functions with the GPD model. The new method for determining the optimal POR of outdoor design conditions was examined and the case for Tianjin was studied. The result shows that the optimal POR for Tianjin is 17 years. (C) 2014 Published by Elsevier B.V.