By means of experiments, the laws of deformation response of joint of cast irons are studied under normal step exciting force. It was discovered that the curve of step response consisted of two parts of instantaneous elastic part and long -lasting viscous part. Also, under action of different amplitude of exciting force and pretightening force, viscous part was about a third of total deformation and the proportion remained unchanged. The phenomena could not be expressed by Kelvin -Voigt model, which often is used in modeling joint. Therefore, a new modeling method of normal characteristics of joint was presented. The experiment principle and experiment setup were introduced in study first, and then characteristics of deformation response of joint were analyzed under step exciting force. Problems existed in Kelvin -Voight model and three -element model, which often is used in viscoelasticity analysis, were studied in modeling joint. On that basis, three -element model was modified, and then a four element model was presented for modeling joint. Finally, step responses of the new model are compared with experimental results, which indicates that the proposed model can more accurately reflect normal viscoelastic properties of joint.