A new model was proposed to calculate the viscosity of fluids under the pressure ranging from 0.1 to 110MPa by improving the Tait equation, in which the viscosity mu of liquids was linked with activation volume V. The model with two adjustable parameters alpha and beta was applied in calculating viscosities for alkane, aromatic and alcohol family at high pressure. Results show that calculated values of viscosity are in good agreement with the experimental ones, and the average relative deviations for alkanes, aromatics and alcohols are 0.56%, 0.31% and 0.66%, respectively. Besides, the errors correlated by the model proposed in this paper were equivalent to the ones from the pure empirical Tait equation, and obviously superior to those from Eyring equation.