The elastic moduli associated with the a‐b plane are obtained by the pulse echo overlap technique from room temperature to 200 °C for Bi2Sr2CaCu2O8 single crystals. The temperature dependence of elastic moduli, especially C′, exhibits an anomalous variation. The change of C′ with decreasing temperature is an overall trend of softening over a wide temperature range, and the maximum amplitude of softening reaches about 24%. This behavior is strikingly different from that of other moduli measured in the experiment. Three obvious softening minima of C′ around 65, 90, and 150 °C reflect the property of lattice instability and a shear‐type phase‐like transition. The relation between the lattice instability and the softening of the elastic mode is discussed.