A {CaCo3}-containing sandwich-type polyoxometalate, Li2Na2Ca3(H2O)16{Co3(H2O)[Ca(H2O)5PW8O31](PW9O34)}·16.5H2O (1), was synthesized by using a trivacant phosphotungstate to react with Co2+ and Ca2+ by a conventional aqueous solution method. 1 was characterized by single crystal X-ray diffraction, powder X-ray diffraction, IR spectra, elemental analyses and TG analyses. Structural analysis reveals that 1 is composed of trivacant and a tetravacant phosphotungstates to trap a {Co3} cluster and a Ca2+ cation. The obtained sandwich-type polyoxoanion was further connected to form a two-dimensional expanded structure via the Ca2+ linkers. This work highlights an efficient method to simultaneously introduce both alkaline-earth and transition-metal cations into polyoxotungstates.