The defects, bubbles, and dirty spots in the optical elements of a laser interferometer introduce coherent noise similar to the "bull's eye ring" into the interferogram. To solve this problem, a coherent noise suppression method based on an extended light source of multimode fibers is proposed. The proposed method uses the extended light source to suppress coherent noise and selects the optimal multimode fiber core to ensure a favorable interference fringe contrast. Then, a composite speckle suppression technique is employed, and the speckle contrast is reduced by introducing the multimode fibers and rotating ground glass to suppress the speckle noise generated by the mode interference of the multimode fibers. Simulation analysis and experimental verification are carried out in an interferometer with a diameter of 300 mm. The following observations can be made from the results. Under the conditions of a vertical planar Fizeau interferometer with a diameter of 300 mm, a cavity length of 500 mm, and a camera exposure time of 5 ms, an interference fringe contrast higher than 0.75 can be ensured by keeping the core diameter of the multimode fibers in the proposed light source system between 0.52 mm and 1.70 mm. The speckle contrast of the final image obtained by the interferometer is maintained at about 0.04, with a minimum of 0.044. The extended light source of multimode fibers further improves the uniformity of the light source, contributing to a more uniform imaging background in the interferometer. The experiment proves that the proposed method can effectively suppress the coherence noise in the interferogram.