Abstract: The Asian-Pacific Oscillation (APO) is a phenomenon in which the temperature changes out of phase over the East Asia continent (15°-50° N,60°-120°E) with the North Pacific (15°-50° N,180°-120°W) in the upper troposphere. The APO index reflects the Asian-Pacific tonal thermal contrast. The performance of the fast coupled version of the LASU/IAP climate system model FGOALS_gl in simulating the upper troposphere temperature and the APO index over the 20th century is evaluated. Compared with the ERA-40 rcanalysis data, it is shown that the model performed well in simulating the climatology and the dominant modes of the upper troposphere temperature.However, the results show that the simulated APO index failed to capture the descent trend after 1960s over the East Asia continent as indicated in the ERA-40 data. Based on the power spectrum analysis,the 2-3 a variability of the model APO index is equivalent with that in the rcanalysis but the 5-7 a variability is weaker. Despite several regional departures,the large-scale circulation over Asian monsoon section related with the APO index is well reproduced in the model. A comparison among the 20th century simulations shows that external forcing could change the interannual variability of a couple system. The natural forcing causes a spectrum shift to low frequency and the anthropogenic forcing does inversely. Natural forcing and anthropogenic forcing can play different roles in different periods. It seems that anthropogenic forcing could limit the interannual variability of APO and enhance the interdecadal variability. The dominant mode of the upper troposphere temperature in the model is modulated by ENSO and further impacts the interannual variability of APO. The defect of the model in the ENSO simulation may be an important limitation to reproducing the upper troposphere temperature and the variability of APO index.