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In this work, electric machine of implantable rotary blood pumps (RBPs) of axial-flow type was theoretically investigated. Electromagnetic coupling of rotor and stator in axial-flow RBPs was described with the Maxwell’s equations of the classical electrodynamics given quasi-magnetostatic approximation of electromagnetic field in the simplified model consisting of permanent magnet and conductive loop rotating in free space with one spatial degree of freedom. Additionally, relative field error, created by the neglect of geometric deviations introduced through manufacturing tolerances, was estimated for a typical axial-flow RBP. Upper limit of error introduced by the simplifications was estimated less than 0.2 %, leading to the finite accuracy of the description and clearly determining the influence on the control signal. Based on the presented description and additional engineering considerations, the electric machine of axial-flow RBPs was defined as a three-phase non-salient pole synchronous machine with a permanent magnet rotor. Two key features were shown: a) unlike the conventional electric motors, signal of back electromotive force tends to be a sinusoidal waveform in any construction of axial-flow RBP with significant non-magnetic gap; b) the optimal waveform of control signal in this case is sinusoidal. Initial design and control parameters of the electric machine in axial-flow RBPs can be accurately determined with presented theoretical description. Based on the description, control system of an axial-flow RBP with the optimal waveform of control signal can be developed.