Abstract: By using the Fourier optics, a theoretical model based on the far field is put forward for the coherent addition of multi-ultrashort pulses. The impacts of all kinds of errors on the temporal and spatial properties in the far field are studied for a system with a 50 cm aperture and 1 ps ultrashort pulses. Results show that the phase-delay error must be less than 0.63 and the rotation errors must be less than 0.37 μrad and 0.34 μrad in the direction of x and y, respectively, with a criterion that the encircled energy in the 1×diffraction limit not less than 0.9 of that in the ideal case. Results also show that the residual chirp factor must be less than 1.32 and l.52 without and with 0.63 phase delay, respectively, with a criterion that the temporal stretch in the far field less than 0.2