The effect of water resistance concentration on water resistance and water resistance strength has been studied through the water resistance finishing treatment of woven cotton fabrics by pressing-drying-heating method. The water resistant agent used in the study is Phobotex RHP Hydrophobic Agent. The water resistance and water resistance strength of pre and post-treated fabric samples are evaluated according to standard AATCC 22-2017. Research results show that the higher the concentration of Phobotex RHP Hydrophobic Agent, the higher the ability to resist water resistance and the greater the water resistance strenght of the fabric. Conversely, when the concentration of water repellent is increased to a certain limit, the water resistance of the treated fabric reaches 100% and reaches the saturation level. Research has determined that the concentration of water repellent within the manufacturer's recommended range provides the best water resistance for cotton fabrics after many washing cycles. The research results are a scientific basis of practical significance for manufacturers to choose the right concentration of water repellants for each purpose.