An ultrasonic waves test scheme for three-point bending load of reinforced concrete I-beam was developed and implemented. Ultrasonic waves data were collected under 21 load levels, Taylor expansion was applied to eliminate the influence of high order signal and extract the multiorder content of amplitude change of wave, and an approximate linear curve between amplitude first-order change coefficient and stress change was established. The research shows that under the condition of three-point bending stress, the first-order change coefficient of the wave amplitude of reinforced concrete I-beam can be fitted linearly with its stress change, and the goodness of fit is higher than 0.99.