Theoretical research of dynamics of electron flow with virtual cathode in lowvoltage vircator under external harmonic force has been held in this work. The behavior of electron flow was considered within the framework of both stationary analytic model and numerical simulation of electron flow nonstationary processes. The increasing of lowvoltage vircator output power while increasing external force has been shown in both cases. The results of theoretical research let us to explain physical phenomenon leading to output power increasing.