The contrast of long and short vowels and the acoustic features and physiological rules of long vowels in Hongshuihe (lit. Red Water River) dialect of Zhuang are investigated in this paper. The results show that in isolated words read at normal speed, there are evident differences in the durations of long and short vowels. But different vowels behave differently. Due to phonetic physiology, when long vowels are pronounced, the tongue is higher for high vowels, lower for low vowels, more front for front vowels and more back for back vowels. The pitch and power of short vowels are higher than for corresponding long vowels. Finally, physiological explanations for the above phenomena are given.