The active engine mount (AEM) is developed in automotive industry to improve overall NVH performance. The AEM is designed to reduce major-order signals of engine vibration over a broad frequency range, therefore it is of vital importance to extract major-order signals from vibration before the actuator of the AEM works. This work focuses on a method of real-time extraction of the major-order acceleration signals at the passive side of the AEM. Firstly, the transient engine speed is tracked and calculated, from which the FFT method with a constant sampling rate is used to identify the time-related frequencies as the fundamental frequencies. Then the major-order signals in frequency domain are computed according to the certain multiple relation of the fundamental frequencies. After that, the major-order signals can be reconstructed in time domain, which are proved accurate through offline simulation, compared with the given signals. To verify the real-time performance of the method, a hardware-in-the-loop testing system based on MATLAB xPC target is established. LMS Data Acquisition System is adopted to track rotating speed online and extract major-order signals offline, the results of which are considered as the comparison with the online results from the hardware-in-the-loop testing system. It can be found that the method features high accuracy in extracting order information online with a reduced computational burden, therefore it satisfies the requirement of the real-time control of the AEM.