The invention relates to a biomimetic fish, in particular to a biomimetic fish for detecting river water composition. The biomimetic fish for detecting the river water composition can detect the composition of river water and is high in practicability and multifunctional. The biomimetic fish for detecting the river water composition comprises a water quality detector, a rack, gears, first oscillating rods, first connecting rods, fins, a motor, a rolling cylinder, a contact rolling wheel, a second connecting rod, a disc, a first joint rod, a second oscillating rod, a first supporting rod, a sliding sleeve, a lifting rod, a second joint rod, a first water pipe, a water pump, a second water pump, a tank body, a fish tail, a bearing pedestal, a biomimetic fish body and a rotating shaft; and the motor is connected with the rear wall in the biomimetic fish body. According to the biomimetic fish for detecting the river water composition, the effects of detecting of the river water composition, high practicability and multifunction are achieved; and the biomimetic fish is ornamental and can explore whether river water is polluted or not according to detected results.