A freshwater biomonitor system is designed to perform in situ monitoring of the organism behaviors in freshwater. The freshwater biomonitor measures and analyses different types of behaviors of aquatic species based on the registration of changes in a high frequency alternating current, rapid and sensitive behavioral responses of the indicator species, caused by the movements of organisms in the test chambers, monitoring changes of toxicity in the freshwater finally. Experiments show that the freshwater biomonitor is easy to use and stable in working, and it can perform in situ measurement to changes of toxicity in freshwater accurately.