Abstract: Objectives In order to achieve the accurate control of variable-mass unmanned surface(USV) vehicle under different payload delivery tasks, this paper proposes an adaptive surge control method for variable-mass USV with uncertain variation in mass and draft. Methods Taking draft and its higher-order terms as independent variables, the analytical expressions of mass and each hydrodynamic derivatives are analyzed. In view of the high correlation between the motion state and draft of the variable-mass USV, a super-twisting sliding mode observer is designed to obtatin the estimated unknown draft and mass, and the finite time stability of the observer is proved via Lyapunov theory. The adaptive surge control algorithm is designed based on the decouped variable-mass USV maneuvering model, and the adaptive control rate and strategy are updated in real time based on the super-twisting sliding mode observer. The overall stability of the adaptive control system is verified by Lyapunov method. Results The simulation experiments of several payload delivery missions of variable-mass USV are carried out. The results show that the proposed draft observation algorithm can realize the accurate observation of the draft and mass. The proposed adaptive surge control algorithm can also achieve the stable tracking of the control target under typical conditions such as mass step change and continuous variation. Conclusions Therefore, the designed control algorithm is suitable for all kinds of control conditions of variable-mass USV.