Underactuated autonomous underwater vehicle (AUV) is a kind of AUV with independent control inputs less than the number of degrees of freedom of motion. Its trajectory tracking problem has increasingly become a research hotspot. This paper proposes a backstepping adaptive sliding mode control algorithm, which realizes the trajectory tracking of underactuated AUV under external environmental interference, and derives it to prove the stability. The algorithm introduces a position error margin, uses the backstepping method to design the desired speed of underactuated AUV, and proposes an antiinterference adaptive sliding mode control, which achieves the convergence of the tracking position error by speed control. Finally, through simulation, the trajectory tracking problem of helix is solved and analyzed by using this method, which shows the timeliness, stability and robustness.