The long distance side-pumped laser fiber (LDSPF) is characterized by its ability for high power pump power coupling, heat management and nonlinear effect suppressing, which is quite potential for high power fiber laser application.A (1+1) LDSPF for tandem pumping was fabricated and tested. Pumped by 1018 nm fiber lasers, 17.4 kW laser output at 1080 nm with a slope efficiency of 82.1% was achieved with this fiber amplifier with backward pumping technique. Linewidth at 3 dB of the laser is 1.3 nm and the ratio of signal light to SRS is around 37.8 dB at the maximum power. The results demonstrate great potential of LDSPF as power amplifier for tens of kilowatts fiber laser.