The dirving moment calculation model of each nozzle segment of the three bearing swivel nozzle(3BSN) was built based on the deflection law model and the given deflection law of the 3BSN. The changing law of the moment produced by each nozzle segment's gravity and gas aerodynamic force to the instantaneous rotation axis with the varying nozzle deflection angle was obtained by respective force analysis for each nozzle segment. The effect of the nozzle geometrical parameters on the moment to the nozzle instantaneous rotation axis was obtained by changing the realizable maximum nozzle deflection angle and the length of the second and third nozzle segments. The maximum moment of the second and third nozzle segments' aerodynamic force to the instantaneous rotation axis increased with the realizable maximum deflection angle of the nozzle and occurred at the location in approximately 70% of the maximum deflection angle. The moment of the second and third nozzle segments' aerodynamic force to the instantaneous rotation axis increaseed with the nozzle segment's length increases.