Based on the analysis of ultrasonic ranging and principle of objective judgment, a fish finder system was designed, using Bluetooth 4.0 low power consumption technology and combining mobile APP with hardware.An integrated chip CC2540 was used to control the receiving and sending signal of the ultrasonic transducers, and to obtain the target echo signal. After A/D sampling, the signal data was packaged in a custom format then sent to cellphone with Bluetooth 4.0 function.Algorithm processing was completed in the mobile terminal to distinguish useful target signal(fish or underwater); and to calculate underwater depth of the target, which could present visually through APP interface of the phone. Finally the feasibility of the system was verified through practical test.