The mechanism responsible for the formation of the low-level jet (LLJ) over the South China Sea during spring and summer of 1998 was analyzed through using the assimilated grid data provided by the South China Sea monsoon experiment (SCSMEX). More analyses were emphasized on a heavy rainfall process in the Zhujiang Delta area between 8th and 10th June 1998 in which a LLJ played a key role. Furthermore, the MM5 meso-scale model was utilized in studying the effects of the weather systems on the LLJ. The results showed that the kinetic energy associated with the LLJ transferred upward when the LLJ formed, which results in the fact that the big velocity of the LLJ expanded upward to higher level. Simulation verifies that a synoptic-scale low pressure in the low troposphere of the west of South China moved toward the east, with a low-troposphere synoptic-scale high pressure in the western Pacific moving toward the west. In this process, the approach of both systems leads to the reinforcement of pressure gradient force between them, making the kinetic energy needed by the formation of the LLJ increase. Through this process the LLJ formed and developed over the South China Sea between the low and high pressures. Because the formation of the LLJ indicates the enhancement of summer monsoon in the South China Sea, the mechanism responsible for formation of the LLJ could be used in explaining an enhancement of summer monsoon in the South China Sea.