Several studies suggested that plasma-activated water (PAW) increases crop yields and confronts drought. This study determined the total phenolic (TP) and total flavonoid (TF) contents of water spinach to elucidate whether PAW induced changes in the antioxidant potential of water spinach planted in soil with and without metal contaminants. PAW was selected as the irrigation water for pot experiments. Results showed that PAW increased the TP of water spinach planted in soil without any contaminants and, to some extent, in Cd-added soils but decreased the TP of those planted in Pb-added soil. PAW significantly enhanced the TF of water spinach planted in Cd-added cultivation soil, but not for Pb-added cultivation soil. Meanwhile, PAW can elevate the TF of water spinach by adding fertilizer and sunlight exposure. This study demonstrated that PAW application could increase TF, powerful antioxidants of water spinach planted with fertilizer and sunlight, in the cultivation soil with Cd, or without any contaminant.