B-box is a form of vocal percussion that imitates rhythms in various types of sound, especially musical instruments. As b-box becoming popular, more and more people want to learn b-box and make their own b-box music. However, not everyone has the talent for generating harmonic b-box music. In this work, we develop an interactive system which helps the user easily compose b-box music given two inputs: An unaccompanied vocal song and a piece of b-box rhythm. The audio signals of the two inputs are analyzed and adaptively matched on the basis of their beats. The state of the art beat detection technique does not perform well on vocal songs. Hence, we propose to partition the song into short segments and estimate the average tempo for each segment so that the adjustment of tempo will not be affected too much by the wrongly detected beats. With the proposed system, people who love b-box or are not familiar with b-box can enjoy producing their own b-box music.