The invention provides a preparation method and application of phthalic acid diester (2-propyl group heptanol), which obtain the phthalic acid diester (2-propyl group heptanol) (DPHP) through a one-step reaction with phthalic anhydride and the 2-propyl group heptanol serving as raw materials. The raw materials selected by in the preparation method and the application are easy to obtain and mild in reaction condition, the postprocessing after reaction is simple, operation is simple and convenient, chemical yield coefficient is high and the like. The preparation method is convenient in operation, saves energy, and is less in environmental contamination. Obtained products have low volatility, good thermal ageing performance, good weather fastness and good anti-mobility, and can serve as a plasticizer in the polrvinyl chloride processing process.