This paper presents a pulse base effect on the Brillouin gain spectrum that can be obtainedby Brillouin optical time-domain analysis (BOTDA), which uses a pump–probe system. Fortheoretical analysis of the pulse base effect, we propose a theoretical model in which theinteraction between the pulse base and probe light is taken into consideration. Strainunderestimation caused by the pulse base is demonstrated by experiments and theoreticalsimulations.