The sound-speed and density contrasts are important factors in estimating the target strength (TS) of moon jellyfish (Aurelia aurita). In this study, the sound-speed and density contrasts were measured using time-of-flight and neutral buoyancy methods, respectively. The sound-speed contrast of A. aurita was from 0.9966 to 1.0031 (mean${pm}$SD, $0.9999{pm}0.0017$) and no distinct differences in temperature or pulsation activity and weak were found. The density contrast was from 0.9994 to 1.0004 (mean${pm}$SD, $1.0000{pm}0.0002$). The density of A. aurita was substantially different but the density contrast of A. aurita was shown to be similar to that in the sampling location. The results can be used to estimate of TS of A. aurita by acoustic model.