We propose a phase difference measurement method based on digital holographic microscopy. The three-dimensional topography characterization of highly transparent phase-type samples is achieved through digital holographic reconstruction. The environmental refractive index is calculated in combination with the phase difference, and the solution mass concentration is inverted based on the refractive index?mass concentration model. Under ideal conditions, this method achieves a refractive index measurement resolution of 10^-6 level and has the capability for non-contact and real-time dynamic measurement. In addition, the phase difference method features a simple device and low cost, and has unique advantages in engineering applications. Through comparative experiments, it is verified that the mass concentration measurement results of the phase difference method and the fluorescence spectrophotometer have consistent trends, and the former has a higher signal-to-noise ratio. This method is applied to the study of the adsorption mechanism of microplastics on polycyclic aromatic hydrocarbons. Focusing on the adsorption processes of polypropylene (PP), polyethylene (PE), and polystyrene (PS) in the typical polycyclic aromatic hydrocarbon naphthalene solution, the morphologies of the three microplastic particles and the mass concentrations of the naphthalene solution that changed with adsorption time are quantitatively measured. The adsorption effect is studied based on the parameters of the adsorption dynamic equation. The experimental results show that there is a significant correlation between microplastic particles with different three-dimensional morphologies and adsorption performance, confirming the application value of the proposed method in the multi-dimensional characterization of complex liquid phase systems.