A model of coherent beam combination (CBC) of fiber laser with 7-channel and 37-channel is establishedaccording to Fraunhofer diffraction theory. The far-field intensity distribution and beam quality of coherent beamcombination with different duty ratios and tilt angles are simulated. The influences of tip/tilt phase error, duty ratioand the number of sub-beams on CBC of far field beam quality are analyzed. Simulation results show that the farfield intensity in the Airy disk decreases with increasing tilt angle, which means degraded combination effect. Thepower in the bucket (PIB) of the far field decreases more rapidly with tilt angle increased in a large- scalecombination, which means that the accumulated effect of tip/tilt phase error with the number increase of sub-beamscan appear. In order to improve the efficiency of CBC, the tip/tilt phase error and piston phase error should becontrolled simultaneously.