Abstract: During the repairing process of S31803 duplex stainless steel，the zones of original weld which near to the repaired weld was reheated again．And the ferrite phase in this zone has a further transformation to austenite phase，which leads to the decrease of the ferrite phase proportion and can not even meet the standard requirements．Through a series of tests and analyses，some control measures of phase proportion in this zone were made，which can play a guiding role in welding procedure qualification tests and welding production of S31803 duplex stainless steel．