Freqnency standard is a key device in many civil and military applications.Saturated absorptian can minimize the Doppler expansion and lead to high performance frequency standard.An experimental equipment to study the saturated absorption in R_b vapor cell is described in this paper.Experimet result shows that saturated absorption has a optimal temperature under which biggest absorption occurs.For a 300 μm vaper sell, 50 ℃ temporature is appropriate.Experiment also shows that the redative intensity of crossover resoncmce of the 300 μm cell is smaller than the 50 mm cell.Analysis and explanation about the phenomenon is made.The experiment and the conclusicn is of reference value for the research of atom clock.