A design study of the multistage axial helium compressor of a 300MW nuclear gas turbine is presented in this paper. To design the helium compressor, we apply a new velocity triangle, which is different from that of the conventional air compressor. And the new velocity triangle is of large flow coefficient and negative pre-swirl etc. Comparing with the design based on conventional air compressor technique, the new helium compressor stage loading is increased by nearly three times when inlet flow coefficient increases from 0.6 to 2.04, thus stage numbers could decrease from 16 to 6. The higher loadings result in exacerbated corner separations. An increased reaction can relief the three dimensional separation and achieve the expectant stage pressure ratio. The ratio of the maximum thickness to the chord length of the rotor blade from hub to shroud is optimized and three dimensional profiling is applied. In such a way, the separation in the highly loaded helium compressor channel can be controlled and the optimum efficiency achieved. The performance investigation of the designed stage shows that the highly loaded design has acceptable stall margin.