By the far field excitation and measurement, propagation loss of wire’s surface plasmon is experimentally investigated with structure of branched-routing silver nanowire. The experimental results show that the propagation loss depends on the exciting wavelength. The coefficient of propagation loss is measured as 0.115 μm-1 at 632.8 nm and 0.0923 μm-1 at 780 nm. The propagation loss is smaller at longer wavelength, that is, SPP can propagate further at long-wavelength laser exciting. The measurement contributes to optimizing plasmonic devices based on silver nanowire waveguides.