Purpose: Long distance runners’ fatigue may be postponed by supplementing the branched chain amino acid. Method: 16 medium and long distance runners are classified at random into an experimental group (BCAA) and a control group. During the experiment, the two groups of runners will have sports of the same load. After six weeks, increasing aerobics load experiment (4mmol/L of blood lactate) and simulated 3km and 5km tests under extreme load are carried out on the treadmills respectively. Blood is sampled before the sports, immediately after the sports and 30 min after the sports for measuring the level of blood glucose, blood lactate and alanine. Research results: in extreme load sports, after the BCAA group has sports, their blood glucose immediately after having sports falls far below the level of the resting state (P<0.05); the level of blood glucose of the control group immediately after having sports and 30 min after having sports falls far below the level of blood glucose of the resting state (P<0.05). The alanine concentration of the BCAA group immediately after having sports and 30 min after having sports rises far above the level of resting state and is significantly higher than that of the control group (P<0.05); that of the control group immediately after having sports is significantly lower than the level of the resting state (P<0.05). The blood lactate of the BCAA group after having sports is significantly lower than that of the control group (P<0.05). Conclusion: BCAA may help to postpone the long distance runners’ fatigue and eliminate the fatigue after having sports.