To study the effects of the positive focal shift on the impulse coupling coefficients about the CO2 pulse laser focus on the aluminium target at air ambient. By changing the distance from the focus to the target, the relation between the positive focal shift and the impulse coupling coefficients is achieved. The experimental result indicates that coupling coefficients declined from 4.48×10^-5 N?s/J to 1.68×10^-5 N?s/J, when the focal shift changes from 0 to 20 mm.While the focal shift increase between the 20 mm and 35 mm,the coupling coefficients rise and reach 2.21×10^-5 N?s/J, then decrease to 1.91×10^-5 N?s/J at the 45 mm.The two-dimensional model theory of laser supported detonation wave interacting with solid target was used to calculate the coupling coefficients. The trends of theory and experiment are same.