An automatic recognition algorithm of testing bulk modulus by making use of variant transverse wave is proposed. Use two straight beam probes to measure the direct longitudinal wave’s arriving time as well as the delay wave’s arriving time caused by variant transverse wave for rodshaped testpieces of different materials. According to this two parameters, longitudinal and shear wave velocities and bulk modulus can be calculated of the materials. Design an automatic recognition algorithm to find the target points instead of using lock gates after analysis the influence of different material properties and dimensions. The results show that the uncertainty is less than 1% when testpieces are metal materials such as steel and aluminum whose lengthdiameter ratio is about 5, and the uncertainty is less than 3% of nonmetal testpieces like plexiglass and nylon the same size.