In order to meet experimental requirements on high parameter plasma research, the injected power of the counter injector is expected to increase at a fixed beam energy of 50 keV for deuterium neutral beam. Three candidate schemes are compared to assess the possibilities of raising the injected power. Considering safety and economic factors, raising the electric field in the first gap is employed by adjusting the voltage gradient on the accelerator. The experiments show that the optimum perveance is increased from 2.2 to 2.7 μP by changing of gradient grid voltage from 0.84 to 0.78 Vacc. The ion beam power is promoted by about 50% at 3.0 μP, but beam transmission efficiency decreased to 60% at 3.0 μP. Thus, the injected power is boosted about 25%.