## 1 Introduction  
The marine surface vessel (MSV) mainly relies on the guidance system to perform the required control scheme, such as path following, target tracking and path tracking, in which the influences of wind, currents and waves on the ship, which are simply considered in [1; 2; 3; 4]. The path following control aims to control the ship to reach and track a predetermined parameterized curve at a certain velocity without time constraints [5]. In the last few years, the control problem of path following for the underactuated MSV has attracted extensive attention in the control field [6; 7; 8]. The guidance law is an important component of the path following problem, which aims to compute the desired heading angle for the subsequent controller design by processing path information. The classical guidance laws consist of the line-of-sight (LOS) guidance, constant bearing guidance, and pure pursuit guidance [9]. Especially, because the LOS guidance law has simple and intuitive characteristics, it has been widely used in the path following control of MSV. At an early stage, the classical LOS guidance law for reducing the cross-track error was presented in [10]. However, although the classical LOS guidance laws are simple and effective, they have limitations for the vessel, subject to external disturbances. Then, the integral line-of-sight (ILOS) guidance law was developed by introducing the integral actions into the LOS guidance law, which can eliminate the drifting effect [11; 12]. Due to the complexity of the environment and the requirement of the operation accuracy,it is necessary to not only consider the tracking performance of the desired path, but also to ensure that the position errors of the ship during navigation does not have large jitter, otherwise it is not conducive to the accurate path tracking of the ship. In addition, in order to navigate safely, the actuator saturation of the underactuated MSV should be limited. Otherwise, the control performance of the system will deteriorate and lead to the instability of the control system [13]. For the problem of position error constraint and actuator saturation, in [14], a path-following controller based on the error-constrained line-of-sight (EC-LOS) guidance law was presented for an MSV. An asymmetric time-varying barrier Lyapunov function (BLF) and a smooth hyperbolic tangent function were proposed for a fully actuated MSV to address the asymmetrical input and output constraint in [15]. In the case of actuator saturation and external disturbance, a path following control scheme for a MSV with a prescribed performance was presented in [16]. In [17], a control scheme was presented for the underactuated MSV, which considers not only the position error constraint, input saturation, external disturbance and model uncertainties, but also time-varying sideslip. However, the convergence time of the above literature is asymptotically convergent, which implies that exact convergence cannot be guaranteed in finite time, and the influence of the convergence time is not considered. In order to obtain a high-performance solution, this paper proposes an FT-LOS guidance law, which can ensure that the position errors will not exceed the error constraint, and converge into a small neighborhood around zero in finite time. In addition to the requirements of the error constraints and input saturation, it is also important to consider the effect of convergence time on system performance. The finite-time controller makes the system errors converge in finite time, thereby obtaining faster convergence rate and better disturbance rejection properties. The research on the finite-time control of the underactuated MSV focuses on trajectory tracking control problem. In [18], a finite-time disturbance observer was presented to reduce the environment disturbances, which has the advantages of fast convergence speed and strong robustness.