MnOx was supported on peracetic acid modified coconut shell activated carbon (MnOx/AC) via facile in-situ redox reactions and used for catalytic oxidation of HCHO which is a major indoor air pollutant. The MnOx/AC catalysts were characterized by XRD, Raman, XPS, BET and in-situ DRIFTS technologies. The prepared MnOx was delta-type structure with poor crystalline. With the addition of methanol as a reducing agent during preparation, the MnOx/AC-methanol catalyst exhibited superior catalytic activity and stability toward HCHO oxidation at room temperature. The HCHO removal efficiency was kept nearly at 100% for a reaction time of 1000 min and remained highly stable even after six repetitive cycles. The rich Mn3+ and surface oxygen groups were beneficial for the catalytic activity. The in-situ DRIFTS results showed lower accumulation and faster decomposition of intermediates on the surface of the MnOx/AC-methanol catalyst, which may account for its superior catalytic efficiency and stability.