Technique for passive phase locking of fiber lasers by an all-optical feedback loop is studied theoretically and experimentally. The experimental setup of coherent beam combination of three fiber lasers in one dimension is built, and phase-locking of three fiber lasers is achieved and stable interference patterns are observed. The influence of duty ratios on the interference patterns in the far field is studied. Main factors that affect the cavity loss are analyzed. Random peaks corresponding to different wavelengths are found in the spectrum of coherent output laser. By using a narrow bandwidth filter, the spectral range of coherent output laser can be restricted. However, the phenomenon mentioned above still exists. And theoretical analysis is given to explain these random wavelengths existing in this kind of technique for passive phase-locking.