The ignition threshold of laser supported detonation wave (LSDW) can be determined based on the component variation of plasma in the laser absorption zone with laser power density. A kinetic pendulum equation is presented for being adopted in the experiment of impulse coupling between laser supported detonation wave and aluminum target. Then considering the Jouguet relation between impulse and specific heat ratio of medium in the laser absorption zone (plasma and gas), the relation between specific heat ratio and laser power density can be obtained for analyzing the result of two-dimensional hydrodynamic simulation. The specific heat ratio decreases while laser power density increasing. Finally, considering the difference between plasma and gas, the component of plasma and gas can be determined. The laser supported detonation ignition threshold is calculated in the space interval from (1.62±0.01)×108 W/cm2 to (2.10±0.07)×108 W/cm2. By comparing to the ignition threshold determined by the impulse-coupling coefficient, there is an ideal concordance in these two ways.