Coherent beam combination of Yb-doped fiber laser array is studied theoretically and experimentally by spatial filtering to passive phase locking. The principle of optical feedback phase locking based on spatial filtering is analyzed, and the relations of Strehl ratio in far field to wavelength and spectrum bandwidth are studied. The coherent beam combination of four Yb-doped fiber amplifier array is achieved experimentally, and stable interference pattern is obtained. The Strehl ratio in far field is 0.77. In the output spectrum, the bandwidth of single longitudinal mode is about 9.1 GHz consistent with the theoretical calculation.