Swirl injector with adjustable backhole length was analyzed to suppress high-frequency combustion instability in Liquid Rocket Engines. In order to analyze the effect of variable backhole injector as an acoustic absorber, backhole injector was regarded as a quarter-wave resonator. As a result of theoretical approaches and acoustic tests, backhole injector with adjustable length could decrease the unstable modes of combustion chamber. And the damping efficiency was estimated by measuring damping rates experimentally.