A method for constructing a host strain capable of highly expressing human serum albumin (HSA) includes: designing an optimized gene encoding the HSA; constructing a recombinant expression vector; and transforming the recombinant expression vector into a Pichia pastoris host strain through electroporation to produce the host strain. Through the molecular-level optimization design for the full sequence of the target gene, a target gene fragment most suitable for the expression in Pichia pastoris is acquired. At the cellular level, the internationally latest Pichia pastoris expression system host strain CBS7435 is introduced to express the gene for HSA. Pichia pastoris transformants carrying the exogenous gene with a high copy number are selected with the geneticin (G418) resistance gene to construct an engineered strain for expressing recombinant HSA at a high level. The method can be used to prepare HSA with a purity of 99% or more.