The invention discloses a method and a system for automatically detecting the detergent concentration of a washing machine, and the washing machine. The method comprises the following steps: after water enters the washing machine automatically, adding a preset amount of a detergent according to the amount of the water; after the detergent is mixed with the water uniformly, starting a spectrograph which is arranged in the washing machine to acquire spectroscopic data of washing water in a washing barrel and generating a spectrogram corresponding to the spectroscopic data; analyzing the spectrogram to obtain the concentration of a surfactant in the washing water; presetting a first concentration value, and calculating the using amount of the detergent which needs to be supplemented according to the concentration, the preset amount of the detergent and a preset first concentration value, and adding the detergent in the corresponding using amount into the washing barrel. According to the method and the system for automatically detecting the detergent concentration of the washing machine, and the washing machine, the concentration of the surfactant in the washing water can be detected accurately by mounting the spectrograph on the washing machine so as to judge whether the using amount of the detergent is suitable or not according to the concentration of the surfactant.