A theoretical investigation of dynamic behaviors of helical geared multishaft rotor systems is performed in this paper. A modal synthesis is developed to calculate free and forced vibrations for geared rotor systems including more than two shafts and gears. Degrees of freedom can be reduced to save the computing time in this method. Results of reduced model are compared with those of full degrees of freedom model, to obtain the acceptable reduction of degrees of freedom without significant loss of accuracy in predicting free and forced vibrations. Then, with the help of this method, dynamic behaviors of helical geared multishaft rotor systems are investigated. Parametric studies are conducted to reveal the effects of several system parameters on the vibration characteristics. Results show the reliability and accuracy of the modal synthesis as well as its limitations of calculating responses due to high-frequency excitations, and provide some references to designers attempting to obtain desirable dynamic behaviors.