Fine particle pH plays an important role in aerosol chemistry. Understanding of influencing factors and evolution process of fine particle pH is helpful to facilitate the improvement of air quality management strategies. Here, we performed simultaneously field measurement at Mt. Tai and the surrounding urban site during the summer of 2018, and analyzed comprehensively fine particle pH and its driving factors based on the sensitivity tests. The results indicated that ambient temperature (Temp), atmospheric relative humidity (RH), TNH₃ (= NH₄⁺ + NH₃) and SO₄²⁻ were the dominant factors for fine particle pH. The switch between RH-sensitive and Temp-sensitive regimes with variation of RH at the mountain site implied that fine particle pH was more sensitive to RH at Mt. Tai. Elevated TNH₃ could not sufficiently raise fine particle pH to neutral at relatively high SO₄²⁻ levels, whereas had a significant effect on fine particle pH when SO₄²⁻ concentration was relatively low. Considering the current levels of TNH₃ in summer of the North China Plain (NCP), fine particle pH was more sensitive to SO₄²⁻ at present. However, the sensitive factor of fine particle pH in the future is expected to shift from SO₄²⁻ to TNH₃ with declining SO₂ emissions and increasing NH₃ emissions, leading in turn to the evident increase of fine particle pH. Elevated fine particle pH and abundant TNH₃ may enhance secondary inorganic aerosol production, and thus effectively synergetic control of SO₂ and NH₃ is urgently needed to be considered in the NCP and even in other regions of China.