The aerodynamic shape, overall design and operation principle of stratospheric airships are different from the conventional aircrafts such as airplane and missile, and the flight mechanism. The dynamics characteristics of the stratospheric airship are different from the conventional aircrafts. Therefore, the inherent mechanism and characteristics of the stratospheric airship need to be further investigated. The linearization equations of motion for stratospheric airship were derived by using the “small disturbance” theory, which could be divided into longitudinal motion and lateral-directional motion. The fight modes and mode parameters were defined, and an analytical method was proposed to analyze the flight dynamics. Flight modes of the stratospheric airship were investigated based on eigenvalue and eigenvector of the state equations. Results show that the longitudinal motion includes three modes: heave mode, surge mode and pendulum mode, whereas the lateral motion includes another three modes: yaw mode, sideslip mode and roll oscillation mode.