The invention relates to a preparation method of medium-viscosity poly alpha-olefin base oil. The preparation method comprises the following steps: under conditions with a temperature of 20 DEG C-120 DEG C, reaction pressure of 0.1 MPa-1.4 MPa and a volume ratio of alpha-olefin/a solvent of (1:0)-(1:3), catalyzing 1-decene or 1-decene and alpha-olefin of C8-C12 for oligomerization for 2 hours-12 hours by using an alkaline-modified AlCl3/Al2O3 immobilized catalyst; and filtering the catalyst after stopping reaction, washing by distilled water, distilling under the normal pressure to remove the solvent, distilling under reduced pressure to remove un-reacted alpha-olefin to obtain alpha-olefin oligomer base oil, wherein the yield of the poly alpha-olefin is greater than 60 omega%, the movement viscosity of the oligomer at 100 DEG C is 10.0mm<2>/s-25.0mm<2>/s, and the chlorine content of the oligomer is less than 2.0 omega%.