The invention discloses a liquid level detecting system. The system comprises a photoelectricity collecting module, a threshold value computing module, a variance computing module, a state judging module and a liquid level moment computing module which are connected in sequence. The photoelectricity collecting module is used for carrying out real-time collecting on photoelectricity signals. The threshold value computing module is used for storing a high-state threshold value, a low-state threshold value, a maximum stable variance and a minimum stable variance. The variance computing module is used for computing the variance of a real-time photoelectricity value with a mobile variance method. The state judging module is used for judging the state of the photoelectricity value. The liquid level moment computing module is used for obtaining the moment when liquid to be detected goes down to the photoelectricity collecting module. The invention further discloses a liquid level detecting method. The mobile variance method is used for detecting the reaching moment of a liquid level, in different environments, for different liquids to be detected, the liquid level is detected accurately, and anti-jamming capability is high.