To overcome the crack problem of thin wall structure of harmonic gear transmission─ flexspline, we develop a transient dynamic analysis model for B1-50-80 harmonic gear transmission. Based on large displacement and large strain theory, the transient stress state of the flexspline is analyzed under maximum rated conditions. The distribution law of transient stress is obtained when the amplitude of stress histories on flexspline gear changes in sine form and the change process of stress of flexspline gear ring are from top to bottom. We observe that over-stress of flexspline on tube gradually decreases along the axis and the stress distribution between flexspline and flexspline gear ring becomes a certain slant due to force irregularly on flexspline tooth in the axial direction. The maximum Von Mises stresses appears in the contact regions between flexspline gear ring and wave generator. Therefore, cracks will originate from the dedendum regions of flexspline, then extend along the axis, and finally expand along 45°.