By the aid of VC++, the influence of thickness monitoring error and witness glass inhomogeneity on the thickness monitoring of optical films is analyzed. It is found that both thickness monitoring error and witness glass inhomogeneity have effect on the thickness monitoring of optical thin films, and the inhomogeneity of witness glass increases with increase of coating layers. Then a multi-layer coating is fabricated and its monitoring curve is analyzed, it is shown that the theoretical monitoring curve which takes thickness monitoring error and inhomogeneity of witness glass into account is close to the experimental monitoring curve, and such a conclusion is got that thickness monitoring error and witness glass inhomogeneity are the important factors which cause the deviation of experimental monitoring curve from theoretical one. In the end, the calculation method of theoretical monitoring curve is proposed, in which thickness monitoring error and witness glass inhomogeneity error are taken into account. The results are of importance for the automatic thickness monitoring of thin films, especially of non-quarter wave coatings.