In the early 21st century, the Lancang River Basin (LCRB) experienced an abrupt decrease in summer precipitation. Based on this conclusion, rainfall spatiotemporal variation characteristics and their effects on the frequency of droughts and floods were analyzed. The main results are as follows: The significant rainfall decreasing trend mainly occurred in June over the middle and low reaches of the LCRB, while the weak opposite trend mainly occurred in July over the middle and low reaches of the LCRB. These variations in characteristics led to the shortening of the LCRB wet period, which further contributed to an increase in droughts in June and the occurrence of heavy rain in July. The facts above indicate that in recent decades, the LCRB has experienced a notable increase in the risks of droughts in presummer and floods in midsummer. A further study of large-scale atmospheric circulation variation possibly associated with the remarkable rainfall decrease in June indicated that northerly anomalies existed over the lower troposphere of the LCRB, which hindered water vapor transport from the Indian Ocean and the Bay of Bengal (BOB). Moreover, cyclone anomalies in the higher troposphere of the LCRB formed an anomalous convergence, which was detrimental to the occurrence of rain. In addition, obvious northerly and westerly anomalies were present in the low and high troposphere over the BOB, respectively, indicating the monsoon in the BOB was weak, which hindered water vapor transport from the Indian Ocean and BOB as well. Due to such large-scale atmospheric circulation anomalies, the LCRB June rainfall has decreased significantly since 2002.