The causes of a disastrous rainstorm in Shandong Peninsula on 27 July 2006 are analyzed. It is shown that the rainstorm was resulted from four mesoβ convective cloud clusters. The inverse trough of the tropical cyclone “Kaemi,” the westerly trough at high level and the subtropical high interacted mutually. The southerly jet formed between tropical low and subtropical high, and the water vapor channel into the rainstorm area was set up. The warm and wet air transportation increased the temperature and humidity in the lowlevel atmosphere over the heavy rain area, so the atmosphere became convectively instable. On the left front of the lowlevel southerly jet, the convergence, positive vorticity, and upward movement developed so that positive khelicity increased. On the right backside of the southwesterly jet in the front of the high level trough, the divergence and negative vorticity developed so that the negative khelicity increased tempestuously, which produced pumping effect to the lowlevel atmosphere, resulting in the convergence and increased upward movement at low levels. The convective instability energy was triggered and released, so that the mesoscale convective cloud clusters were produced and rainstorms occurred.