The principle of the viscosity measurement of liquids with a forced vibration torsion pendulum is studied in this paper-The parameters of the testing system was determined by a 3-parameter model containing a Voigt unit,and the feasibility of the corresponding 4-parameter model of the measuring system,in which a liquid sample has been introduced to measure its viscosity,is then verified-It is proposed,based on the experimental results,that the measuring system is reliable-