The improved semi-analytical finite element(SAFE) model for longitudinal mode guided waves in pipeline is built to research the source influence on longitudinal mode guided waves. Comparing with previous SAFE models, the improved SAFE model can compute longitudinal mode guided waves independently, which significantly improves the computational efficiency. The mathematical model for source influence on longitudinal mode guided waves is built based on the normal mode expansion technique. The model shows the amplitudes of longitudinal mode guided waves depend on the wave structures of guided waves and the characteristics of sources. The numerical calculations and experiments are done to research two questions, one is the amplitude relationships of different longitudinal mode guided waves excited by the same source, the other one is the amplitude relationships of a certain mode longitudinal mode guided wave excited by different sources. The experimental results accord with numerical ones, which shows the correctness of the mathematical model. The conclusions lay the foundations for exciting a certain longitudinal mode guided wave in pipeline by designing suitable transducers.