The invention discloses a method and system for feeding a chemical to boiler water. The method comprises the steps of acquiring index data in a boiler water sample, wherein the index data comprises the phosphate concentration, the pH, the conductivity and the water supply amount; acquiring a change rate of the unit load; judging whether the change rate is within a set threshold range or not; when a judgment result shows that the change rate is within the set threshold range, performing PID control on the boiler water by taking the phosphate concentration as a main control parameter and taking the pH and the conductivity as auxiliary control parameters, and generating a chemical feeding pump first openness signal; when the judgment result shows that the change rate is not within the set threshold range, performing PID control on the boiler water by taking the water supply amount as a control parameter, and generating a chemical feeding pump second openness signal; generating a control signal according to the openness signals; and controlling the rotating speed of a motor of a chemical feeding pump according to the control signal. By adopting the method and system disclosed by the invention, the stability and reliability of the control parameters are ensured more comprehensively, the delay of the control process is reduced, the control precision is improved, and fluctuations and errors, which are brought about by one signal sampling error, of the single parameter are avoided.