In the formation control of unmanned surface vehicles (USV), the control performance is easily affected by the initial state of the system, internal and external disturbances. In this paper, a leader-follower formation control strategy, which combines with the fixed-time disturbance observer, is proposed to deal with the problem of USV formation control. First of all, a fixed-time sliding mode tracking control(FTSM-TC) strategy is proposed, which ensures the leader USV can track the desired trajectory within a fixed time. Secondly, a fixed-time disturbance observer(FTDO) is designed to estimate unmodeled dynamics and external disturbance, which achieves accurate estimation of disturbances. The proposed FTDO-based formation control(FTDO-FC) strategy can maintain a stable desired formation, and enables that the formation control system can converge in a fixed time. Simulation results show that the proposed control strategy can effectively handle unknown disturbances, and the convergence time is independent of the initial state of the system.