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We present a high-brightness, nanosecond pulsed blue laser source at 476.8 nm through efficient quadruple-harmonic generation from a thulium-doped yttrium lithium fluoride (Tm:YLF) master oscillator power amplifier operating at 1.9 μm. The fundamental-frequency stage produces 42 mJ pulses at 1907.3 nm with a narrow linewidth of 0.19 nm at 1 kHz. Through cascade second-harmonic generation using low-walk-off lithium triborate crystals, we achieve 10.52 mJ blue laser pulses with 16.1 ns duration, corresponding to a peak power of 0.65 MW and exhibiting excellent energy stability of 0.47%. The system maintains exceptional beam quality (M 2 x = 1.46, M 2 y = 1.27) at maximum output power, attributed to the negative thermal-optical properties of the Tm:YLF crystal, end-pumped amplification architecture and optimized nonlinear conversion. This work demonstrates a compact and efficient route to high-brightness (~2.49 GW·cm–2·sr–1) pulsed blue laser emission, which is particularly suitable for advanced marine scientific applications including underwater LiDAR and communication systems.