Abstract: The thermodynamic analysis on the system of Fe(Ⅱ)-Mn(Ⅱ)-Zn(Ⅱ)-CO₃^2--NH₃-H₂O is carried out. The relationship between pH value and the total concentrations of Fe(Ⅱ), Mn(Ⅱ) and Zn(Ⅱ) is obtained, and the field of coprecipitation of Fe(Ⅱ), Mn(Ⅱ) and Zn(Ⅱ) is obtained at a suitable pH value. The results show that the field of coprecipitation is included in the field of precipitation of Zn (Ⅱ), while the field of precipitation of Zn(Ⅱ) is determined by the reciprocal ship of N_T and C_T. Furthermore the field of precipitation of Fe(Ⅱ) is related with C_T only. Confirming the coprecipitation of Fe(Ⅱ), Mn(Ⅱ) and Zn(Ⅱ) it has important significance to prepare Mn-Zn soft magnetic ferrite ultrafine.