A comprehensive system with a high speed is built for imaging the terahertz (THz) surface plasmon polaritons (SPPs). Both the amplitude and the phase information of the focusing THz-SPPs excited by a semicircular plasmonic lens are achieved by using this system. The amplitude images present the focusing profiles of the THz-SPPs with different frequencies and the phase images reveal the Gouy phase shift as the THz-SPPs evolving through the focus. The simulations are also performed and a good agreement between the experimental and simulated results has been found.