The invention discloses a preparation method of a Yb3+ doped silica fiber preform mandrel. The preparation method comprises the steps that silicon oxide powder doped with rare earth and co-doping agent Al, P and the like is prepared; decarbonization, dehydroxylation and ball-milling treatment are carried out; high-temperature purification is carried out; the purified powder biscuit is smelted at a high temperature to form glass, and the glass is prepared into the mandrel. The Yb3+ doped silica fiber preform mandrel with the diameter range of 3-18 mm and the length range of 50-300 mm can be prepared, the doping evenness of the mandrel is quite good, and the refractive index fluctuation of the mandrel delta n is smaller than 2*10<-4>. Correspondingly, the mandrel and a pure silica glass sleeving bar are adopted, the preform is prepared through a pipe-bar method, after optical fiber is formed through drawing, the refractive index evenness delta n of the optical fiber exceeds the detection limit of an instrument, and the back bottom loss of the optical fiber at the 1200 nm position is lowered from 0.5-1 dB to 0.05 dB/m and reaches the loss level of MCVD technology.