The invention relates to an automatic assembling device for a lock cylinder. The automatic assembling device comprises a frame and a workbench arranged on the frame. A big rotary table is arranged on the workbench, a clamps for holding a tumbler is arranged on the big rotary table, a tumbler mounting component, an glass bead spring mounting component, an glass bead mounting component, an locking plate spring mounting component, an lock piece mounting component, an tail piece mounting component, an locking plate mounting component, an key mounting component and a discharging component are arranged along the circumference of the big rotary table, and all the components are connected with a main circuit respectively and are controlled by the main circuit. Structure design of the automatic assembling device of lock cylinder is reasonable. Since the components of the lock cylinder are fed, detected, assembled and discharged automatically under electric control, producing efficiency and product quality are improved, labor force of workers is greatly reduced, and producing cost is saved; requirement of large amount producing is adaptably met, and market competitiveness of a company is increased.