In order to explore the relationship between the tenderness and the apoptotic activity during yak meat postmortem aging. The longissimus dorsi muscles of the yak treated by conventional chilling and rapid chilling were taken as experiment objects. The changes of shear force, myofibrillar fragmentation index (MFI) and caspase-9,3 activity at different time points in postmortem were measured. The results indicated that rapid chilling accelerated the rate of decrease of temperature in the muscle center and decreased the rate of drop of pH value in early postmortem aging. The rapid chilling led to no significant change in activity of caspase-9 and MFI within 6 h in postmortem (p>0.05), which delayed the peak times of pH value, shear force and caspase-9,3 activities. In addition, the shear force value in rapid chilling group was higher than the value in conventional chilling group, and the MFI was lower in rapid chilling group compared to conventional chilling group. The MFI in rapid chilling group and conventional chilling group increased by 56.59% and 58.60%, respectively, at 168 h in postmortem aging, the difference was significant (p<0.05). The above research shows that the increase of temperature reduction rate during rapid chilling significantly slows down the reduction rate of pH, so that the apoptotic activity was inhibited in the early postmortem aging, leading to the weak degradation of myofibrillar protein, which reduced the tenderness of beef and prolonged the aging time of muscle.