Abstract: A mesoβ scale analysis is made on the torrential rain process occurred in Jinan in 18 July 2007 based on the 1°×1° NCEP reanalysis data， the surfaceobserved hourly rainfall data and the FY2C IR satellite images. The physical mechanism for the formation and evolvement of the surface mesoβ scale cyclone is revealed. Furthermore， the impact of multiscale merging processes on the torrential rain is especially studied. It is found that the sinking cold flows diffused southwest in the low level at the left rear of the developed mature MαCS，which enhanced the baroclinicity near suface under the joint effect with the southwest warm moist flows strengthened continually since afternoon with the result that the cyclonic disturbance over the surface convergence line was intensified and the mesoβ scale cyclones are generated and developed quickly. The mesoscale convective systems experienced a multiscale cloud merging process including four stages from the mesoγ scale convective cell， to the mesoβ scale and then the mesoα scale convective cluster， and finally to the mesoscale convective complex system. During every stage， the surface mesoβ scale cyclones had played an important role， which were an organizer for both the mesoβ scale convective cluster and the mesoα scale convective cluster. With the encounter and merger of mesoβ scale cyclones， the convective cluster organized by more than one mesoβ scale cyclones was intensively developed and the rainfall was enormously strengthened. During the more than one hour before the torrential rain event 〖JP3〗occured in Jinan， there were the mesoβ scale superlowlevel jets emerging continuously in the southeastern boundary layer， which stimulated the echo cells to emerge successively in this area and grew into a strong echo band during their moving northeast. When the strong echo north of Jinan moved southwardly and encountered with this strong echo band， they combined with each other and developed quickly, which resulted in the severe rainfall.