Provided are an underwater resin matrix light high-strength composite material and a preparation method thereof. The underwater resin matrix light high-strength composite material comprises the following components by weight: 80 to 100 parts of epoxy resin, 8 to 12 parts of curing agent, 5 to 20 parts of flexibilizer, 0 part to 5 parts of thinning agent, 1 part to 2 parts of coupling agent and 60 to 100 parts of cenosphere. The preparation method includes (1) adding the epoxy resin, the thinning agent and the flexibilizer into a reaction kettle, controlling temperature to be in a range from 60 to 80 DEG C and conducting pre-polymerization reaction for 30min to 60min; (2) adding the curing agent into a prepolymer in the reaction kettle to obtain resin glue solution; (3) conducting surface modification processing on the glass cenosphere through silane coupling agent; (4) adding the resin glue solution and the surface-modified glass cenosphere into a vacuum stirring machine to be mixed; (5) adding the mixed materials in the step4 into a three-roller mixing mill for mixing; (6) placing materials obtained in the step5 into a vacuum drying box for vacuum defoamation; and (7) pouring materials in the step6 into a mould for hot-pressing solidification formation. The underwater resin matrix light high-strength composite material is low in density, high in pressure resistant strength, simple in process and low in cost.