The invention discloses an underwater very low frequency (VLF) broadband sound source, which is composed of a shell, a vibration component, a guide component, a support and seal component, a magnetic circuit component, a seal and an electric interface. The underwater VLF broadband sound source is applied to the fields of underwater sound detection, marine geological prospecting, underwater target simulation and the like as a detection target or a test sound source. For the purpose of improving response flatness, broadening bandwidth, strengthening functional reliability, and increasing sound source level, in the underwater VLF broadband sound source, the vibration component is subjected to modality and quality control, thereby realizing light weight, large flexible rigidity and small parasitic oscillation; the working frequency lower limit of a system is reduced by flexible support, thus the working band is wider and flatter; the reliability of a coil and the fidelity of signals are improved by the guide device with limit and low friction; the magnetic driving force is improved by the twin-coil combination and the optimization of a strong-magnet closed magnetic circuit; and the underwater VLF broadband sound source is internally provided with a rubber air bag, thus being simple in structure, realizing self-adaption hydrostatic pressure compensation, and being convenient to use.