A mathematical model of the deformation properties of synthetic fibres in the region of nondestructive mechanical effects is proposed based on a normalized arctangent function of the logarithm of the reduced time, which significantly increases the time, load, and deformation intervals in which calculation prediction of the viscoelastic processes of synthetic fibres is performed. Methods of determining the viscoelastic characteristics as parameters of this model based on the results of brief tests in simple relaxation and creep regimes were developed based on the proposed mathematical model. The methods of considering the irreversible pseudoplastic component of deformation in mathematical modeling of the viscoelastic properties of synthetic fibres increase the reliability of predicting complex deformation modes.