The physical processes in the generation and the evolution of copper plasma induced by a high power nanosecond laser pulse are investigated with time-resolved spectrum. In the solid-gas-liquid phase transition, the intensity of reflected beam evolving with time markedly reveals the absorption of laser energy in the laser-copper interacting zone. So the phase transitions on the surface of copper target result in the fact that intensity of reflected beam evolving with time obviously presents a double-peak structure. Meanwhile, the appearance times of the plasma-gas-liquid phase transition on the target surface is advanced with the increase of peak-power of laser pulse. Therefore, these transient properties can be an efficient approach to diagnosing the phase transitions of copper target when the laser irradiates the target surface.