A practical path planning algorithm for unmanned surface vehicle has been developed. The negative influence generated by the environmental factors such as currents has been minimised for path planning. Anisotropic fast marching algorithm is used as the base algorithm to search for an optimal trajectory in a synthetic vector field. A novel obstacle/expectation decision process is proposed to analyse the planning space with the changes of USV’s position. A 4-regime risk strategy is proposed to assist with the path planning in a dynamic maritime environment.