The bottom ash is increasingly used as a substitute aggregate material in road construction in China, and road salting is the major salt source in groundwater. Continuous rainfall releases soluble salts from the bottom ash subgrade into the surrounding soil and groundwater, resulting in potential hazards. Different methods were employed to simulate and collect runoff water during rainfall events, including batch leaching test, dynamic leaching test and constant head test, to assess environmental impact of bottom ash as road basement materials under continuous rainfall conditions. This study simulated the seepage of bottom ash backfill roads under different rainfall intensities, rainfall times, and rainfall pH values. A comprehensive sampling and laboratory testing program was undertaken to characterize the environmental impact of soluble salts from bottom ash. The obtained results reveal that the leaching concentrations of Cl⁻ and SO₄²⁻ exceed the limit specified in the class V standard of surface water, which are 2.06–2.17 times and 1.08–1.25 times, respectively. By examining the long-term environmental influence under the condition of continuous rainfall, the leaching of Cl⁻ mainly occurs in the early leaching stage, and the maximum leaching concentration reaches 19,700 mg/L. The release concentration of Cl⁻ begins to be lower than the class V standard of surface water when continuous rainfall approaches the total rainfall for 13 months. The cumulative release of Cl⁻ in the bottom ash is 2.8–5.4 mg/g. Both rainfall intensity and rain pH affect the release of Cl⁻. The obtained results derived from the constant head tests indicate that stagnant water caused by rainfall deteriorates the release of soluble salt into the groundwater in only 1 day, especially at the early stage of 12 h. This work provides some basic information about how to minimize damage to the surrounding environment caused by the leaching of salt in bottom ash.