  Recently, great progress has been made in the entanglement of multiple
photons at various wavelengths and in different degrees of freedom for optical
quantum information applied in diverse scenarios. However, multi-photon
entanglement in the transmission window of green light under the water has not
been reported yet. Here, by combining femtosecond laser based multi-photon
entanglement and entanglement-maintaining frequency upconversion techniques, we
successfully generate a green two-photon polarization-entangled Bell state and
a green three-photon Greenberger-Horne-Zeilinger (GHZ) state, whose state
fidelities are 0.893$\mathbf{\pm}$0.002 and 0.595$\mathbf{\pm}$0.023,
respectively. Our result provides a scalable method to prepare green
multi-photon entanglement, which may have wide applications in underwater
quantum information.
