The invention discloses an underwater self-contained sound intensity instrument and a sound intensity measurement method. A sound intensity probe is installed at the lower end of a storage bin bottom seat, and the interior of a storage bin is provided with a lower partitioning plate and an upper partitioning plate. A signal conditioning module is disposed on the lower partitioning plate, and a microprocessing and storage module and a power module are disposed on the upper partitioning plate. A data reading interface and a storage battery charging interface are disposed on an upper cover plate of the storage bin, and are sealed through O-shaped rings. An output end of the sound intensity probe is connected with an input end of the signal conditioning module. An output end of the signal conditioning module is connected with the microprocessing and storage module. The power module supplies power to the sound intensity probe, the signal conditioning module and the microprocessing and storage module at the same time. The data reading interface is connected with the microprocessing and storage module, and the storage battery charging interface is connected with the power module. The instrument can enable the instant sound intensity, mean sound intensity and complex sound intensity data to be stored in an instrument without a cable.