Substrate layer is the key factor determining the purification efficiency of road runoff by dry grass swales. Six dry grass swale columns with different substrate components were constructed for simulation, and the artificial runoff containing representative polycyclic aromatic hydrocarbons (PAHs) was used as influent water. The performance of different simulation columns for the removal of PAHs was analyzed, and the mass balance was performed, in order to evaluate the effects of the presence and types of exogenous organic matters in substrates on the removal efficiency of PAHs and their fate. Results show that the average removal rates of NaP, PhA, and FlA were 69.0%-87.5%, 33.7%-72.9%, and 90.8%-96.8%, respectively. The dry grass swale with a shallow substrate layer was less conducive to the removal of low molecular weight (LMW) PAHs. The addition of exogenous organic matters in substrates was beneficial to improve the removal efficiencies of influent PAHs. During the five-month operation period, with the increase in the molecular weight of PAHs, the accumulation of PAHs in substrates increased and the degree of biodegradation was correspondingly decreased. Under the operation conditions close to the actual situations, the microbial degradation rates of NaP, PhA, and FlA were 62.1%-76.6%, 32.2%-42.9%, and 17.1%-36.6%, respectively. Biochar and woodchips could effectively improve the degradation rates of PAHs, and both of them can be used as exogenous organic materials in practical application of low impact development (LID) facilities, while woodchips are more economical.