The quality of medical ultrasound imaging significantly depends on the performance of ultrasound transducer. During transducer fabricating, a key task is to determine appropriate materials and parameters of acoustic matching layers. To meet the requirements of the high frequency ultrasound transducer, in this paper the acoustic parameters of the composite of RTV615 silicone rubber mixed by alumina powder with different densities were investigated by theoretical models and experiments. The acoustic parameters include the sound speed, acoustic impendence and attenuation. The results show that the acoustic impendence of the 7.93% volume fraction of alumina doped composite is increased by 52.94%, up to 1.56×106 Pa·s/m at 20 MHz, which makes it more efficiently match with human tissue. It is a potential passive material for high frequency acoustic matching layer and transducer lens.