Objective To investigate how interference from motor and cognitive tasks affects the postural stability of older adults during stair descent. Methods A total of elderly subjects were recruited. Using the Vicon infrared motion capture system and Kistler force plate, the kinematic and dynamic data were collected simultaneously during stair descent under three conditions: single task (ST), motor task (MT), and cognitive task (CT). Dynamic stability of the body during stair descent was assessed using the margin of stability (MoS) algorithm. Repeated measures ANOVA was applied to compare differences across conditions. Results Compared to the ST condition, both step speed (P<0.001) and step frequency (P<0.001) were significantly reduced under MT and CT conditions. Step length (P=0.037) was also significantly reduced under the CT condition compared to those under the ST condition. In the aspect of anterior-posterior dynamic stability, compared to the ST condition, centroid velocity (P<0.001) and extrapolated centroid position (P<0.001) were significantly increased, while MoS (P<0.001) was significantly decreased under both MT and CT conditions. In comparison to the CT condition, centroid displacement (P=0.011) and velocity (P=0.014), as well as extrapolated centroid position (P<0.001), were significantly greater those under the MT condition. In the aspect of medial-lateral dynamic stability, compared to the ST condition, centroid displacement (P<0.001) was significantly reduced under MT and CT conditions Additionally, the extrapolated centroid position (P=0.001) was significantly reduced, and MoS (P=0.038) was significantly increased under the MT condition compared to those under the ST condition. Conclusions Older adults adjust their gait in response to dual-task interference during stairs descent by ‘slowing down step speed, reducing step frequency, and shortening step length’. This adjustment helps maintain medial-lateral dynamic stability, but significantly decreases anterior-posterior dynamic stability, thereby increasing the risk of falls.