A new approach to synthesize self-doped colloidal Cu3-xP NCs with controlled size and localized surface plasmon resonance absorption is reported. These Cu3-xP NCs show ultrafast exciton dynamics and huge optical nonlinearities due to plasmonic resonances, which afford the first demonstration of plasmonic Cu3-xP NCs as simple, effective, and solution-processed nonlinear absorbers for high-energy Q-switched fiber laser.