Water caltrop (Trapa taiwanensis Nakai) was used as the raw material to investigate the effects of drying treatments on the antioxidant activities and phenolic compounds of water caltrop. For methanolic extracts from fresh, freeze dried, and hot air dried samples, at 10 mg/mL, the scavenging ability on 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals was 87.2%, 61.4% and 52.1%, respectively. At 10 mg/mL, the reducing power was 1.63, 0.81 and 0.52, respectively. At 40 mg/mL, the chelating ability on copper ions was 29.6%, 14.8% and 34.3%, respectively. The IC50, half inhibition concentration is the concentration, at which the radicals or ions was scavenged or chelated by 50%. The total phenolics and flavonoids contents were higher in the fresh samples, while the IC50 values were lower. The HPLC analysis showed that four phenolic compounds (gallic acid, quercetin, ferulic acid, and hydroxycinnamic acid were found in water caltrop), and that only had three phenolic compounds (gallic acid, quercetin, and ferulic acid were found after drying). While the freeze-drying treatment could preserve more antioxidant activity of the water caltrop pulps than hotair drying treatment, different drying water caltrop pulps exhibited different antioxidant mechanisms.