This paper is a part of systematic studies on the isomorphous series of wolframite. The experimental results show that the frequencies and the adsorption intensity of several absorption bands of the wolframite exhibiting in the region of medium and far IR spectra are linearly varied with respect to the relative contents of FeWO4 and MnWO4. The correlation regression equations as well as diagrams between frequencies and x(Fe/Fe Mn)are given in the paper, and the reason for the frequency change and the absorption intensity change with respect to x has been discussed.