The invention discloses a preparing method and application of a sandwich type electrochemical luminescence immunosensor for detecting marine pathogenic bacteria. The method is characterized by comprising the steps of obtaining an immunomagnetic bead solution the surface of which is covered with marine pathogenic bacterium antibodies by immobilizing the marine pathogenic bacterium antibodies onto the surfaces of aminated magnetic beads; synthesizing multifunctional oxidized graphene through chemical bonding of the marine pathogenic bacterium antibodies and an electrochemical illuminant; conducting electrode pretreatment and immobilizing immunomagnetic beads onto the surface of a magnetic glassy carbon electrode; finally adsorbing the marine pathogenic bacteria to be detected and the multifunctional oxidized graphene onto the surface of the magnetic glassy carbon electrode in sequence. The obtained sandwich type electrochemical luminescence immunosensor serves as a working electrode, a platinum electrode is taken as a counter electrode and an Ag/AgCl electrode or saturated calomel electrode is taken as a reference electrode to calculate the accurate concentration of the marine pathogenic bacteria in a sample solution to be detected. Detection speed is high, the sensitivity and accuracy of detection results are high, and specificity is high.