The goal of this study is to investigate the performance of a small vertical-axis wind turbine at an environment with the turbulence intensity more than 30%, particularly on the influence of the vertical wind, the vertical angle, the wind with the unknown direction, the horizontal wind speed, and the turbulence intensity on the power output, which are seldom reported before. The results show that more than 90% of the power is generated when the vertical angle is less than or equal to 45 degrees. The vertical wind speed has the obvious influence on the power when the horizontal wind speed is between 5 m/s and 8 m/s. The percentage of the power generated by the wind with the unknown direction decreases from 31.1% to 8.4% as the horizontal wind speed increases from 4 m/s to 9 m/s. The efficiency is over 40% only when the horizontal wind speed is over 8 m/s. The higher turbulence intensity increases the power at the lower wind speed, but decreases the power at the higher wind speed. Furthermore, the results can be used as a reference for the improvement of aerodynamic characteristics, efficiency, CFD simulation and the location selection of a vertical-axis wind turbine.