The invention discloses a method for detecting dissimilar material pipeline sectional area loss based on an ultrasonic guided-wave technology, which belongs to the field of nondestructive test of a pipeline. The method is characterized in that an incident signal form is selected, measurement is carried out to obtain a cross-sectional area of the to-be-measured pipeline, and the result is recordedas S0, the cross-sectional area of the dissimilar material pipeline sectional area loss position which is required for being detected in the pipeline is recorded as S1, the density of the pipeline andthe dissimilar material at the to-be-measured sectional area loss position can be recorded as Rho0 and Rho1; the young modulus and Poisson's ratio of the pipeline and the dissimilar material at the sectional area loss position are recorded as E0, v0 and E1 and v1; the guided wave propagation speed of the pipeline and the dissimilar material at the sectional area loss position is obtained and according to the formula shown as the specification, a guided-wave detection device is employed for detecting the to-be-measured pipeline by using appropriate detection frequency, a guided-wave detectionsignal is obtained, an amplitude value Ar and an amplitude value Ai of a reflection signal and the guided-wave incident signal at the sectional area loss position can be obtained, calculation is carried out according to an equation of T=Ar/Ai, and the cross-sectional area of the dissimilar material at the sectional area loss position is obtained according to the formula shown as the specification.The detection method has important engineering value.