Adaptability to various sea conditions is vital to unmanned surface vessels (USVs). This paper aims to reveal influence of sea conditions on tracking performance. The USV controlled by PID tuned for the baseline sea condition is studied. A three degree of freedom dynamic model is built， and a new evaluation index of control energy consumption accounting for propeller propulsion and steering mechanism operation is designed other than the conventional tracking accuracy. Simulation results demonstrate the variation trend of the tracking accuracy and control energy consumption with sea conditions under the typical straight line navigation case. The performance is significantly affected under high sea conditions， with root mean square error increasing 188 times up to 158.207m and control energy consumption increasing 3 times up to 46.916 kWh. The quantification of the tracking accuracy and control energy consumption with sea conditions provides reference for the design and control of USVs.