Abstract: The model for laser irradiation transmission in far-field was established to determine optical system effective aperture and estimate truncation energy for a synchronous three-channel laser warning system. The numerical relation of far-field spot radius, single channel entrance radius and relative energy difference in different channels was stimulated. The results reveal that the relative energy difference in different channel is independent of the energy density in the far-field spot center, in which the maximum ratio is located at the spot rim. When the maximum relative energy difference is limited, the allowable single channel radius is increased with the farfield spot radius, but the truncation pulse energy or power is reduced. When the maximum relative energy difference is 1%, and the far-field spot radius is 2.5m,4.0m and 7.5m, the allowable single channel radius is 3mm, 5mm and 10mm，the biggest truncation pulse energy is 1.1410-5J 7.5410-6J 2.6810-6J，and the biggest truncation pulse power is 1.63W,1.08W and 0.38W.