In view of the low accuracy of the traditional transmission turbidity detector in the low turbidity range, a transmission measurement method of variable optical path is proposed, and the interference factors to the transmission light measurement are eliminated. Based on this method, a new type of variable optical transmission turbidimeter is designed. The principle of the instrument and the structure of the sensor are introduced. The design of the optical path structure of the turbidity sensor reduces the volume and cost of the turbidity sensor. The system takes STM32 single chip microcomputer as the core, performs circuit control and signal processing, and has strong anti-interference ability. According to the ISO7027 standard, the turbidity standard solution is configured and the related experiments are carried out to verify the reliability and stability of the self-made instrument. The measurement precision of the transmission turbidimeter is improved in the range of low turbidity measurement.