The invention relates to a molecular design phycocyanin beta subunit fluorescent protein combining phycoerythrobilin, fusion protein formed by the phycocyanin beta subunit fluorescent protein and streptavidin, and mutant thereof, having the sequences 1, 2, 3 and 4. The invention also discloses a method for directly using the fluorescent protein fused with the streptavidin for fluorescence immunoassay; and due to thioether bond, 153th cysteine residue conserved by phycocyanin beta subunit not only can be combined with phycocyanobilin (PCB), but also can be combined with the phycoerythrobilin (PEB) through genetic engineering, so that a novel fluorescent phycocyanin can be obtained. The spectrum property of the phycocyanin is completely different from that of the phycocyanin beta subunit fluorescent protein combined with the PCB, has higher fluorescence efficiency, and is convenient for purification due to being provided with His-tag label. Furthermore, the dissolubility can be improved, the fusion protein can be formed by the phycocyanin beta subunit fluorescent protein and the streptavidin, the fluorescent protein can be directly used for fluorescence immunoassay, and the invention is beneficial to the application in various fields.