A recombinant expression vector for producing highly expressed human serum albumin (HSA), a host strain, and a use of the host strain are provided. Through the optimization design for the target gene, the construction of a Pichia pastoris engineered strain carrying the exogenous gene with a high copy number and expressing the exogenous gene at a high level, and the optimization of a pilot-scale fermentation process, an expression level of the recombinant HSA can be as high as 25.82 g/L. A purification process for the recombinant HSA involves only fermentation broth pretreatment and three-step column chromatography, and enables a purity of 99% or more for the target protein. A recovery rate of the entire purification process reaches 60% or more.