Passive phase locking of two multiwavelength fiber lasers has been demonstrated by mutual injection coupling and spatial filtering. Since the fiber loop mirrors are employed as two component lasers’ rear reflectors, multiple wavelengths operate simultaneously owing to their broadband reflection and the compound cavity’s mode selection effect. A single-mode filtering fiber is utilized to collect the expected spatial mode’s energy and feed them back into the coupled laser cavity, and stable and high visibility interference fringes have been obtained by the self-phasing process. A contrastive experiment has been made to investigate its coherent output properties when the rear mirrors are replaced by fiber Bragg gratings. Compared with the ordinary coherent combining of fiber lasers lasing at a single-wavelength, higher output power and efficiency have been obtained, and the cost is that the output beams’ coherence and stability decrease slightly.