As the outputs of resolvers are amplitude-modulated signals containing angular position information, demodulation is necessary to convert analog modulated signals to digital ones. In resolver-to-digital converters, phase locked loop (PLL) is usually adopted as the algorithm for demodulation. However, conventional PLL is only a classical type-II tracking loop which cannot overcome the contradiction between dynamical and steady-state performance. Especially for the measured angular position with high dynamic variations, the demodulation error is often large. In order to improve the demodulation accuracy, the problem of resolver-digital conversion is transformed into the problem of angle tracking control, and a sliding-mode variable-structure demodulation algorithm is proposed. This algorithm can retrain the effects of the model uncertainties on demodulation accuracy caused by speed variation by using switching control term. Experimental results show that the proposed algorithm is feasible.