Using datasets of conventional observations, the regional mosaics of Doppler weather radar and NCEP/NCAR reanalysis, we have conducted a diagnostic analysis of the cause for an extremely heavy rainfall event under binary typhoons pattern occurred in Yangtze River delta from September 15 to 16 in 2016. The results indicate that the heavy rainfall areas of this event locate in the top of inverted trough of the No.1614 TC "Meranti" after it landed, and are corresponding with the convergence zone in the lower and middle troposphere and the divergence zone at the right rear side of high-level jet in the upper troposphere. The overlay of the abundant water vapor around Typhoon "Moranti" and the continually water vapor coming from the periphery of Typhoon "Malecas" over the tropical ocean results in the rainfall intensity in the Yangtze River Delta far greater than that before and after the landfall of Typhoon "Moranti". A northeast-southwest temperature front zone forms along the coast of Zhejiang to Shanghai with the intrusion of cold air behind the high-altitude trough in Jiangsu and Anhui province, and a forcing effect of the special mountain terrain produces in the sea-land junction zone in the north-central part of Zhejiang province, which significantly promotes the development and maintenance of the mesoscale convective system (MCSs). The variation of water vapor flux budget in the four boundary of the heavy rainfall area in Yangtze River delta is well corresponding with to the change of heavy rainfall intensity, and the former varies 6-12 hours in advance with respect to the latter, which can be an indicator for the forecast of severe precipitation.