The Ping Chau Formation at Tung Ping Chau, Hong Kong has retained much of its special lithological layers—locally called “Dragon diving into sea”, mainly containing rhyolite tuff. Based on careful sampling, this paper tests and analyzes major elements and microelements of these layers. The result shows that “Dragon diving into sea” is supersaturated in silicon, alkali-rich, low-potassium, low-magnesium and low-calcium. Therefore, it is a type of weakly peraluminous rock, which has a similar mineral components to low-potassium (tholeiitic) rocks, rich in light rare earth elements and distributed in a negative europium anomaly V-shaped right sloping curve. The research proves that “Dragon diving into sea” is a pyroclastic base-surge formed by phreatomagmatic eruption.