The invention relates to a preparation method of PODE (polyoxymethylene dimethyl ethers) and mainly aims to solve the problems of low catalyst activity and selectivity in technologies for synthesizingPODE from reaction materials including methanol and trioxymethylene in the prior art. According to the preparation method of PODE, methanol and trioxymethylene are used as raw materials and contactedwith a PODE catalyst to react to produce PODE; the catalyst is modified metal ion modified sulfonic acid type polystyrene cation exchange resin including a crosslinked polystyrene framework and a sulfonic acid group; modified metals comprise group VIII elements. Therefore, the preparation method can be used in industrial production of PODE.