The choice of doping radius is the key of ensuring the part-doped fibers to play an effective role in mode selections. This paper begins with the normalized mode distributions. By comparing the intensity distributions of different fiber modes,the use of smaller or larger doping radius is deemed not to be conducive to the fundamental mode selection. And then from the views of saturation gain and mode power allocation,the influences of doping changes on the mode ability to gain access and the ability to power extraction are analyzed in detail. The varying of doping radius changes the interaction opportunities between the mode field and the population inversion,and then changes the mode ability to gain access as well as its mode power output. When the gain doping optimally overlaps with the fundamental mode distribution,and the fundamental mode intensity gets the advantage over other modes,the access gain of the fundamental mode will exceed any other mode,and its power will reach the maximum value. Some quantitative results have indicated that selecting about 0.6 relative doping radius for step-index fiber can help us to efficiently extract the fundamental mode power.