We present a modification to the combined transmission-reflection method, which is frequently used to measure the elastic wave velocities in rocks and minerals at high pressure and high temperature in multi-anvil apparatus. The modification mainly focuses on the adoption of dual-mode ultrasonic transducer and new backing reflector (a molybdenum foil), the former was used to simultaneously measure the travel times of compressional wave and shear wave in the sample, and the latter was used to intensify the reflection signals of the buffer rod. The correction time of the molybdenum foils was determined using the equation of state and highpressure velocities of molybdenum. In addition, the length change of sample under high pressure was also taken into account. Using this method, we simultaneously measured the compressional wave and shear wave velocities in Z-cut a-quartz single crystal to 2 GPa at room temperature, and natural mafic granulite sample to 1.4 GPa and 500 degrees C in a multianvil apparatus. The results of this work agree well with those of previous measurements and demonstrate the feasibility of this new method. We believe that by using this method, more accurate elastic properties of rocks and minerals under high pressure and high temperature can be obtained, especially Poisson's ratio.