  In this paper, we present a theoretical simulation of 87Rb absorption
spectrum in a thermal cm-cell which is adaptive to the experimental
observation. In experiment, the coupling and probe beams are configured to
copropagate but perpendicular polarized, making up to five velocity selective
optical pumping (VSOP) absorption dips able to be identified. A $\Lambda$-type
electromagnetically induced transparency (EIT) is also observed for each group
of velocity-selected atoms. The spectrum by only sweeping the probe beam can be
decomposed into a combination of Doppler-broadened background and three VSOP
dips for each group of velocity-selected atoms, companied by an EIT peak. This
proposed theoretical model can be used to simulate the spectrum adaptive to the
experimental observation by non-linear least-square fit method. The fit for
high quality of experimental observation can determine valuable transition
parameters such as decaying rates and coupling beam power accurately.
