Provided is engineered microorganisms expressing acetoacetyl-CoA reductase variants and a method for improving the yield of PHA. Compared with the wild-type acetoacetyl-CoA reductase represented by SEQ ID NO. 31, the variant has one or more of the following mutations: (1) mutation of valine at position 141 to isoleucine or leucine; (2) mutation of methionine at position 12 to threonine, serine, alanine, leucine, lysine or isoleucine; (3) mutation of isoleucine at position 194 to valine, leucine or methionine; (4) mutation of glutamic acid at position 42 to lysine, glutamine, leucine, aspartic acid, proline, threonine, asparagine, or histidine; and (5) mutation of phenylalanine at position 55 to valine, alanine or isoleucine. The variants and their coding genes can promote the synthesis and accumulation of PHA by the strain and increase the yield of PHA.