To investigate the process and mechanism of the impulse generation of laser ablation target, a comprehensive model including one-dimensional heat conduction and fluid dynamics is developed. The target Al, the most common space debris material, is used based on the proposed numerical model in the calculation of ns laser pulse ablation impulse and impulse coupling coefficient as a function of time. Numerical results agree well with experimental data available. Numerical simulations show that, during the laser pulse the impulse the target gained increases greatly with time; after the laser pulse terminates, the impulse tends to be stable with time. In the process of laser impulse coupling, the laser ablation plasma expands into vacuum and its size also gradually increases; meanwhile the plasma plume absorbs the incident laser energy and leads to the reduction in the laser energy deposited onto the target.