Aiming at the current situation of low endurance and high cost of mobile monitoring equipment for aquaculture, which limits the widespread application of such equipment, a small portable AUV for mobile and fixed-point monitoring of fishery aquaculture is proposed and developed. A seal-like operation mode is proposed, replacing expensive underwater inertial navigation systems with conventional GNSS. The three main hulls are arranged in a pin pattern with a modular structure to increase smoothness. The unpowered diving and floating are controlled by the motor-controlled piston altering the volume of the external oil bladder. With the low-power STM32F407 chip as the control core, FreeRTOS embedded real-time operating system and LabVIEW development environment are employed to develop the AUV water surface autonomous cruise and dynamic obstacle avoidance system, data image acquisition and communication system, and shore-based intelligent fisheries big data interaction interface. The results of energy consumption analysis and simulation experiments show that the AUV has low energy consumption, and the endurance time is about 77 h when carrying a lithium battery of 5.184 kWh, which can perform autonomous cruising. Tests show that the prototype can effectively communicate with the shore-based host computer, collect water quality information and underwater video images. The AUV for aquaculture monitoring has flexibility in carrying sensors, long endurance, low power consumption, low cost, easy maintenance and upgrading, and is able to meet the demand for three-dimensional movement and long-term fixed-point monitoring of various aquaculture.