The invention provides a synthesis method of a phytic acid polymer and an application of the phytic acid polymer in the field of metal surface treatment. The preparation method specifically comprises steps as follows: phytic acid is dissolved in water and prepared to form a phytic acid solution; a silane coupling agent is dissolved in an organic solvent, the phytic acid solution is heated to 55-85 DEG C, the organic solvent solution containing the silane coupling agent is dropwise added to the phytic acid solution while the phytic acid solution is stirred, a thermostatic reaction is performed, and the slight white phytic acid polymer solution is obtained. When the phytic acid polymer solution is applied to metal surface pretreatment, a phytic acid polymer chemical conversion coating can be formed on a metal surface, and the defect of poor corrosion resistance of a film formed merely through phytic acid can be effectively overcome.