The invention relates to a preparation method of a photoelectrochemical immunosensor for detecting cardiac troponin I. Immunoassay and photoelectric test are separated by constructing a split-type photoelectrochemical sensor, so that the photoelectric test and the immune recognition process of biomolecules do not interfere with each other. Bismuth sulfide nanorod modified tungsten trioxide is usedas a substrate material to provide basic photoelectric response, the band gap structures of bismuth sulfide nanorods and tungsten trioxide are matched, and visible light can be well utilized. Then, characteristic immune recognition of an antigen and an antibody is carried out in a 96 micro-well plate, ascorbic acid-coated mesoporous silica packaged by cadmium sulfide is used to label a cardiac troponin I second antibody, and cadmium sulfide is firmly bonded with mesoporous silica through disulfide bonds. After dithiothreitol is added dropwise, the disulfide bonds between cadmium sulfide and silica are disconnected, ascorbic acid is released, so that the photocurrent is improved to different degrees, and sensitive detection of cardiac troponin I is realized. The detection limit is 0.17pg/mL.