Ultra-precision single point diamond flycutting is a novel method to manufacture the microstructures with grooves. In the flycutting process, the cutting force is an important physical quantity. It is closely associated with cutting quality and tool wearing. To develop the cutting regularity, a flycutting force model based on the theory of orthogonal microcutting was presented. The cutting force model on rake and flank face is achieved. The shear angle in flycutting process was analyzed based on the kinematical regularity. The value of the shear angle in specific processing parameters technological conditions was achieved by single-turn flycutting experiments. To verify the flycutting force model, overlapping flycutting experiments with variant parameters feed rates were carried out. The validity of the force model is demonstrated with the accordance of experiments and theoretical analysis.