With the aim to explore the effects of apoptosis on muscle internal environment, apoptosis factors and tenderness during postmortem aging, the longissimus dorsi muscles injected with specific inhibitor cyclosporin A (CsA) of MPTP were taken as experiment objects. The muscle internal environment factors, apoptosis factors and tenderness index were measured. The results indicated that myofiber diameter and cross section area in the CsA group were significantly or extremely significantly larger than those in the control group (P<0.05，P<0.01);after 6h and 72~120h of postmortem, the pH value in the CsA group was lower than that in the control group (P<0.05), and at 6~168h of postmortem, ATP contents in the CsA group were higher than those in control group, specially, after 12hand 72~168h of postmortem, ATP contents in the two groups had significant difference (P<0.05);the degree of MPTP opening in the CsA group was lower than that in the control group (P<0.01);Cyt-c content in the cytoplasm was significantly or extremely significantly lower in the early aging stage, and the result was opposite in the late aging time;caspase-3 activities in the CsA group was lower than those in the control group during the whole time except after 72~120h of postmortem;Hsp27 expression in the CsA group was lower than that in the control group, after 12~24h of postmortem, the shear force of the CsA group was higher compared with the control group. Cyclosporin A could suppress the changes of internal environment, caspase-3 activities, Hsp27 expression level and the shear force and changes of myofibrils and mitochondrial ultrastructure by suppressing the opening of MPTP, the apoptotic process played a significant role in the changes of internal environment and yak meat tenderness during postmortem aging and the changes of internal environment and tenderness could be adjusted by controlling the apoptosis process.