Based on the daily NCEP/NCAR reanalysis data,this paper analyzes the extra-tropical low-frequency oscillation(atmospheric angular momentum budget and mountain torque) and AO(Atlantic Oscillation) indices in winter.Power spectrum analysis and correlation analysis show that the dominant variations of them occur in 30-60 d band,and the variation of atmospheric angular momentum budget has positive correlation with the variation of mountain torque and also responds to the variation of AO significantly.Dynamic diagnostic analysis shows that the mountain torque of the two major NH mountain ranges influences zonal atmospheric angular momentum(zonal westerly),and the subtropical westerly and the north-south westerly dipole structure change,so the strength of AO changes.In comparison with the Rockies,the Himalayas play a important role in AO variation.The dynamic diagnosis of the Himalayas ranges shows that,during the period of high(low) AO indices,the quasi-stationary planetary waves forced by the Himalayas ranges influence subtropical westerly,and the subtropical westerly weakens(strengthens) and the high-latitude westerly strengthens(weakens),so AO strengthens(weakens) and the polar vortex in the stratosphere strengthens(weakens).The strong(weak) center of polar vortex tends to Northeast Asia.