The invention discloses a preparation method for a polymer, belonging to the field of polymers. The method comprises a step of contacting a polydiolefin oligomer with an isoprene monomer used as a polymerization monomer under polymerization conditions in the presence of an organic solvent and a rare earth catalyst, wherein the polydiolefin oligomer is a mixture of an isoprene oligomer and a butadiene oligomer. With the method, polyisoprene rubber with a weight average molecular weight adjustable in a range of 650000 to 2200000 can be prepared. The polyisoprene rubber prepared by using the method has Mooney viscosity of 60 to 85 and is easy to process.