Wet electrostatic precipitators (WESP) have been proven to be a promising technology for the removal of sulfuric acid mist from flue gases. Recent studies indicate that WESPs, when mitigating sulfuric acid mist, can be less effective on systems that burn high sulfur content fuel. This paper reports on a pilot-scale WESP used to investigate SO3 removal with high SO3 concentration. Both mass concentration of SO3 and number concentration of sulfuric acid mist were measured. Two key parameters, i.e., electrical characteristics and gas loading, were studied to evaluate their effects on SO3 removal efficiency under severe corona suppression conditions. Results showed that the maximum corona current was reduced by 83.1% when the WESP inlet SO3 concentration increased from 0 to 5c(0), and the corresponding SO3 removal efficiency decreased from 74.5% to 54.2%. SO3 removal efficiency can be improved by increasing corona power and reducing gas velocity. A comprehensive method was proposed to enhance SO3 removal efficiency, with removal efficiencies greater than 90% using proper electrode configurations and lower velocity under a pilot plant condition. In addition, other challenges including new ultra-fine mist generation, material corrosion and insulator failure are summarized.