Based on the analysis of Pacific SST (sea surface temperature), atmospheric circulation, and precipitation in Qingdao, it is found that there is important relationship between the precipitation in Qingdao and the PDO (Pacific decadal oscillation) index during flood season. When the PDO is in the cold phase, the SST is higher over western North Pacific but lower over the coast of North America and tropical middle-eastern Pacific, the western Pacific subtropical high is weaker and eastward whose ridge line is northward, the East Asian summer monsoon is stronger, and the precipitation in Qingdao during flood season is more. Conversely, the precipitation is less. A new Pacific SST anomaly index, SSTI, is defined, which includes the synergistic effects of the out-of-phase changes of SST anomaly in western North Pacific and the tropical middle-eastern Pacific and also the synergistic effects of the PDO and ENSO. Compared with the PDO index and the SST over western North Pacific and the tropical middle-eastern Pacific, this index has a better correlation with the precipitation in Qingdao during flood season. Generally, the positive SSTI corresponds to the northward position of the ridge line, smaller area and weak intensity of the western Pacific subtropical high during flood season and stronger East Asian summer monsoon, which is favorable for more precipitation in Qingdao during flood season. Conversely, the precipitation is less. So, the SSTI can be used as an indicator for the precipitation prediction during flood season in Qingdao.