Identification of active site is critical for developing advanced heterogeneous catalysis. Here, a nickel/silica (Ni/SiO2) catalyst was prepared through an ammonia-evaporation method for CO methanation. The as-obtained Ni/SiO2 catalyst shows a CO conversion of 96.74% and a methane selectivity of 93.58% at 623 K with a weight hourly space velocity of 25,000 mL.g(-1).h(-1). After 150 h of continuous testing, the CO conversion still retains 96%, which indicates a high catalyst stability and long life. An in situ vacuum transmission infrared spectrum demonstrates that the main active sites locate at the interface between the metal Ni and the SiO2 at a wave number at 2060 cm(-1) for the first time. The interesting discovery of the active site may offer a new insight for design and synthesis of methanation catalysts.