The invention provides an assembling device and an assembling method thereof of a main bearing and a pinion assembly assembling of a shield tunneling machine. According to the invention, the pinion assembly comprises a gear, and the front end and the rear end of the gear are connected with a pinion front end self-aligning bearing and a pinon rear end bearing through a first shaft and a second shaft respectively; the device comprises a counterweight tool, a positioning tool, an assembly tool and a bearing correction tool, by means of the device, the whole pinion shaft and the bearing pedestal are short in installation time consumption; the mounting structure is simple, the assembly precision is high, the mounting process is greatly optimized, the mounting success rate is 100%, and particularly, the assembling device has more obvious advantages in the transmission aspect of the main bearing of the shield tunneling machine.