Blood circle treatment is a new remedy developed for reducing low density lipoprotein and endotoxin of patients′ plasma, consequently, curing or alleviating cardiovascular and cerebrovascular diseases, hypertension and inflammation. However, for there is possibility of plasma leakage before the treated blood is transmitted back to patient and some monitoring procedure is to be maintained. The approach to monitor plasma leakage was presented after a brief introducing of the working principle of the blood circle treatment apparatus. Experiments on ultraviolet absorption spectrum of waste solution of the blood circle treatment apparatus mixed with plasma of different concentration were carried out and, the conclusion that such a solution has an absorption peak at about 278 nm was made. A set of detecting system of plasma leakage suited for the blood circle treatment apparatus was developed with the base of ultraviolet absorption spectrum experiments. In the set double beam scheme was utilized for the optical des