The invention provides a real-time low speed detection device based on virtual sine waves. The device comprises a pulse encoder outputting Ao, Bo, So and Zo four-set pulse signals, a signal conditioning circuit which is a pulse shaping circuit, a sampling conversion module which is a pulse conversion circuit, and a processor, wherein Ao and Bo pulses are equal cycle and equal duty ratio digital pulses, the So pulses are equal cycle and variable duty ratio pulses, the Zo pulses are the zeroing pulses, four input ends of the sampling conversion module are respectively used for receiving the A, B, S and Z four paths of pulse signals, four output ends of the sampling conversion module are for outputting the frequency multiplication pulses AB, the direction signal R, the zeroing signal Z and asine sampling signal S, and an input end of the processor is connected with the four output ends of the sampling conversion module. The device is advantaged in that real-time and rapid rotating speedcalculation is realized, rotating speed measurement precision is higher, moreover, the whole calculation mode employs an interruption and DSP internal counter, the CPU time is not occupied, and the device is especially suitable for directly-driven wind turbines and real-time control systems for speed closed loop.