No Mai-yu rainfall was recorded over the middle-lower reaches of the Yangtze River in the early summer of 1978 and a spell of severe drought lasted from mid-summer to early September.Consequently, there occurred three periods of sustained high temperature from late June to early September which had never been observed in the past hundred years. This happened mainly due to the anomalous development of the subtropical high. A mean pentad subtropical high ring belt at 500 mb surrounding almost the whole northern hemisphere collapsed unstably and then an intense subtropical high cell which had formed over the west Pacific moved westward,intensified and extended pentad by pentad. At the same time, a powerful warm high moved eastward from the Iranian-Tibetan Plateaus. These two high centers then merged into One, intensified and dominated a great part of China, with exceptional high temperatures occurring twice over the Yangtze River reaches once in early July and the other in early September. The anomalous development of the subtropical long’wave systems was also responsible for the sweltering summer in 1978. Analysis indicates that there existed a quasiperiodic change for about one month in the subtropical high and sustained high temperature.