The invention discloses a seabed geomagnetic diurnal variation observation device, belongs to the field of ocean exploration, and solves the problem that the ocean geomagnetic field is difficult to observe in the prior art. The observation device comprises a buoyancy ball, a sensor cabin, an electronic cabin and a release recovery unit; a magnetic sensor is arranged in the sensor cabin; a power supply module, a data acquisition control module, a storage module and a wireless transmission module are arranged in the electronic cabin; the power supply module is used for supplying power to the electronic cabin and the sensor cabin; the data acquisition control module is connected with the storage module and the wireless transmission module; the magnetic sensor in the sensor cabin is electrically connected with the data acquisition control module in the electronic cabin through a watertight cable; and the buoyancy ball, the sensor cabin, the electronic cabin and the release and recovery unit are sequentially connected through cables. The observation device can improve the precision of seabed geomagnetic detection, and is simple in structure and convenient to operate.