Manganese in different forms and valence states in the sample to be tested is converted into manganese ions after a high temperature and pressure reaction in the presence of an oxidizing agent. Manganese ions react with the color developer formaldehyde oxime in an alkaline solution at pH 9.0-10.0 to form a brown complex. The complex has maximum absorption at a specific wavelength, and the total manganese content in the sample to be tested is calculated by photoelectric colorimetry.