First heart sound (Sl) and second heart sound (S2) amplitude ratio (S1IS2) reflects relationship between the cardiac contractility and the peripheral resistance. Between consecutive cardiac cycles，there is almost always a variation in the amplitude of the Sl and S2. The goal of our study was to get an insight into athlete cardiac responses to hypoxia conditions by studying S1IS2 variability. A breath-holding phonocardiogram test (BHPCGT)was designed for inducing a hypoxic condition. When performing the BHPCGT in 116 subjects (71 athletes and 45 non-athletes)， a phenonenon， reversal of S1/S2，was found in 13 samples (6 Full-reversal， 7 Partial-reversal). There were 4 (5.63%) reversed samples from athletes controlled with 9 (20%) from non-athletes. t-test statistic results showed that 51 (71.83%) samples from athletes and 33 (73.33%)企om non-athletes had significant differences (p < 0.05) between the two conditions. We suggest that holding breath does have influence on the function of heart to some extent， and the tolerance to hypoxia might be better in athletes than in non-athletes. The reversal of S1/S2may have a significant value in assessing cardiac reserve， studying cardiac fatigue and observing heart responses to hypoxia. However， further large-sample and multi centre studies are still needed.