The rational design of crystalline materials coupled with polyoxometalate as electrocatalyst is an effective strategy for the sensitive electrochemical detection of H2O2. Here, a new organic-inorganic hybrid, H2(NH4)3[Him][(PIIIO3)2Mo5O15]·4H2O (1) (im=imidazole), was isolated by a one-pot self-assembly strategy. Structural analysis indicates that compound 1 adopts a three-dimensional (3D) supramolecular structure based on the Strandberg-type [(PIIIO3)2Mo5O15]6− clusters linked with protonated [Him]+ cations via hydrogen bond interactions. Electrochemical studies reveal that compound 1 exhibits the multi-electron redox processes ascribed to MoVI centers, and has good electrocatalytic activity in the reduction of H2O2. By utilizing compound 1 as electrode material, the designed electrode showed excellent electrochemical performance towards H2O2 detection, including a wide linear range (50–450 μM), high sensitivity of 0.085 μA·μM−1 and a low detection limit of 3.52 μM, as well as good selectivity and stability.