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A mathematical model for the micro-perforated panel in water medium is estab-lished using mode superposition method,and an equivalent circuit model is acquired based on electro-acoustic analogy method. Effects of the flexible micro-perforated panel (FMPP) and the flexible back panel (FBP) on normal incidence sound absorption coefficient are analyzed. Unlike micro-perforated panel in air medium,the underwater micro-perforated panel cannot achieve any satisfactory sound absorption effect due to insufficient structural impedance. To this end,a strengthened micro-perforated sound absorption structure is proposed. Theoretical results are verified in a water-filled impedance tube. Within the range from 20 Hz to 2,000 Hz the average absorption coefficient of the strengthened flexible micro-perforated panel are improved obviously. The absorption coefficient curve gradually approaches rigid micro-perforated panel as the bending stiffness increases. The flexible back panel can reduce the resonance frequency of the absorption curve and improve the low-frequency sound absorption performance.