NOVELTY - Preparing streptavidin-marked phycourobilin (PUB)-combined phycoerythrincyanin alpha -subunit fluorescent protein by molecular designing comprises: (a) using genetic engineering method to obtain alpha -subunit lyase/isomerase expression plasmid, streptavidin-marked apoenzyme expression plasmid, and phycoerythrobilin (PEB) synthetic plasmid; and (b) transferring the plasmids in a host bacteria, fermenting the engineered bacteria to obtain streptavidin-marked PUB-combined phycoerythrincyanin alpha -subunit fluorescent protein, and purifying.    USE - The method is useful for preparing streptavidin-marked phycourobilin-combined phycoerythrincyanin alpha -subunit fluorescent protein (claimed). The streptavidin-marked phycoerythrincyanin alpha -subunit fluorescent protein can be used as a functional material in biology and medicine, preferably as fluorescent probe for detection.    ADVANTAGE - The method is an environment-friendly production method.    DETAILED DESCRIPTION - Preparing streptavidin-marked phycourobilin (PUB)-combined phycoerythrincyanin alpha -subunit fluorescent protein by molecular designing comprises: (a) using genetic engineering method: cloning the alpha -subunit lyase gene or homologous gene in an expression carrier to obtain alpha -subunit lyase/isomerase expression plasmid; cloning the streptavidin gene and phycoerythrincyanin alpha -subunit apoenzyme or homologous gene in a second expression carrier to obtain streptavidin-marked apoenzyme expression plasmid; cloning the phycoerythrobilin (PEB) biosynthetic enzyme gene or homologous gene in a third and/or fourth expression carrier to obtain PEB synthetic plasmid; and (b) transferring the alpha -lyase/isomerase expression plasmid, streptavidin-marked apoenzyme expression plasmid, and PEB biosynthetic enzyme plasmid in a host bacteria; obtaining corresponding engineered bacteria; using the engineered bacteria in fermentation engineering to produce the streptavidin-marked PUB-combined phycoerythrincyanin alpha -subunit fluorescent protein; after fermenting using conventional protein-purifying technology, purifying corresponding streptavidin-marked PUB-combined phycoerythrincyanin alpha -subunit fluorescent protein.