Stainless steel has always been a focus of researchers because of its excellent adaptability in corrosive environments. A ferritic-austenitic high-silicon duplex stainless cast steel was designed. The pitting corrosion resistance of this stainless steel was characterized and analysed via methods such as microstructure observation, polarization curve determination and immersion corrosion tests to explore the formation and development mechanisms of pitting corrosion. The results demonstrate that the pitting corrosion resistance of high-silicon duplex stainless steel tends to decrease with the heat treatment processing, in the order of as-cast, solution-treated, tempered, and aged states. Pits primarily nucleate at the ferrite-austenite phase boundaries and tend to preferentially attack the austenite phase, forming a porous structure within the matrix. Ultimately, the surface layer sinks inwards or is corroded away, developing into collapse-style pit craters and cave-style pit holes.