A new one-dimensional kinematic model of viscous fingers growth is proposed. This model is based on the assumption of an intermediate layer formation. The shear instability of the flow is developed in this layer due to intensive mixing of liquids of different viscosities. The thickness of the intermediate layer between outer ones is determined by the velocity difference in the layers. The results of the calculation of the growth rate of the fingers in the framework of this model are in a good agreement with the calculations carried out on the basis of more general two-dimensional equations as well as with the known Koval model.