Recently, magnetic force microscope (MFM) for dynamic imaging of AC magnetic field has attracted considerable attention due to its potential applications and special requirements in industry. In this paper, we develop an alternating force MFM technique based on the frequency modulation of MFM tip oscillation, which provides a powerful tool for the development of key technologies in magnetic information storage industry. Different from conventional MFM, the main points of the present work are: 1) the investigation of the frequency-modulation phenomenon; 2) optimization of the MFM tip parameter, and introduction of the MFM signal processing apparatus; 3) observation of the AC magnetic field. For dynamic evaluation of AC magnetic field, we need to theoretically analyze the mechanical and magnetic properties of MFM tips, to technically develop the MFM signal processing apparatus, and to experimentally image the dynamic magnetic signals. Finally, we demonstrate the alternating force MFM technique, which can measure and analyze the nano-scale magnetic domain structures in advanced magnetic materials.