Crop water stress index (CWSI) is an useful tool to quickly assess water status of crop. For usage of CWSI with same baselines for specific crop in different areas under different climate types, we investigated the reason of difference for crop water stress index baseline of summer maize at different treatments. The summer maize field experiments were carried out in red soil areas of south China under different fertilizer treatments during two consecutive years. The results showed that, the morphological indices and leaf age of maize under different nitrogenous fertilizer treatments were different. The low baselines of maize at different growing stage were not same. Fertilization had an effect on low baseline of maize. Nitrogen fertilization reduced the low baselines of maize. The low baseline decreased as the amount of nitrogen fertilization increased. Fertilization also had an effect on upper baseline of maize under severe water stress conditions. The results indicated that, the main reason of difference of CWSI baselines for same crop was the growing conditions and growing stage of crop. The weather condition was the second factor to impact baseline of CWSI.