In order to study the effect of surface coating time on electromagnetic properties and microwave absorbing properties, carbonyl iron was coated with SiO₂ by hydrolysis of tetraethyl orthosilicate (TEOS). The elements and chemical bonds of the samples were characterized by XRF and FT–IR, and the surface morphology of the samples was analyzed by TEM to determine the thickness of the coating. The electromagnetic parameters were measured by a vector network analyzer to study its absorbing properties. The results show that the surface coating thickness is about 40–60nm. With the coating time prolonged, the thickness of the coating layer gradually increases. After simulation, the absorption peak of the electromagnetic wave after coating is moved to a high frequency, band width (R_L≤–10dB) increases significantly. The sample coated for 2h has the smallest reflection loss, the lowest peak value is – 45.18dB, which shows good absorbing performance.