The invention provides internally-arranged FFT on-line frequency detection module application. A frequency detection device comprises a vibration sensor used for detecting vibration frequency of a cabin tower frame of a wind generating set and outputting stimulating signals about detected vibration frequency, an analog-digital converter converting the stimulating signals about the detected vibration frequency to digital signals, a microprocessor which conducts quick fast Fourier transformation on the digital signals received from the analog-digital converter so as to acquire frequency range information about the detected vibration frequency, and confirms vibration amplitudes in a preset frequency range based on acquired frequency range information so as to monitor vibration conditions of the cabin tower frame of the wind generating set. According to the frequency detection device, concerned frequency component energy changing conditions are accurately grasped through on-line FFT analysis, and therefore vibration strength of different parts of a fan is grasped.