This paper is the second part of “Analysis and thinking on the extremes of the 21 July 2012 torrential rain in Beijing”. The first part of “Observation Analysis and Thinking” has analyzed the precipitation features, water vapor conditions, environmental conditions, generation and development of the mesoscale convective systems (MCSs) during the process of the severe torrential rain. As the second part of the series studies, this paper mainly focused on the impact factors of the extreme rainfall, i.e., precipitation efficiency, water vapor, ascending motion and duration and so on, to explore the causes of the extreme torrential rain by using various observation and model data that are often used in operations. The results showed that this extreme severe rainfall event happened under a typical circulation pattern of North China torrential rains. The interaction of high level wind divergence with the low level vortex and wind shear directly triggered the extreme rain event. The comparatively high relative humidity and thick humid layer, low lifting condensation height and free convection height as well as the characteristcs of tropical torrential rains improved the precipitation efficiency of this severely heavy rain process. The extreme high water vapor content (with the atmospheric precipitable water vapor getting to 60-80 mm) and the abnormity of associated physical quantity can be judged as the extreme severe precipitation event. The middle and low level troposphere was instable conditionally, while the upper level had the characteristics of humid neutral stratification. With a medium CAPE value and convective instability enhanced by upper level dry intrusion, the upward motion was intensified. MCS was generated and developed in the warm areas due to the combined actions of vortex wind shear and the terrain in the North China region. The northward jump of vortex and the backward propagation of MCS made the warm area MCS more slowly eastward forming the “train effect” and resulting in the long lasting precipitation. Finally, this paper discussed the methods of forecasting extremely severe rainfalls.