Laser induced impulse coupling is one of the important phenomena in laser ablation to materials and leads to many applications. The amplitude of impulse coupling coefficient is varied with laser parameters on target. As the timescale of repulse pressure is mainly determined by laser duration,laser produced impulse coupling in vacuum is only caused by vaporization contributions,so unsteady vaporization have to be considered in making some calculations on laser induced momentum. The dependence of impulse coupling coefficient to aluminum in vacuum vs. incident laser intensity with different wavelength and pulsewidth are studied by unsteady transparent vapor model. Good agreement between theoretical results and experimental measurements has been achieved.