The invention discloses an automatic fine-adjustment mechanism for fuel capacity control over a fuel injection pump. A fuel capacity control pin (2) is a steel cylinder and is arranged in a hollow bolt (1) in a penetrating mode. The hollow bolt (1) is of a two-segment type step structure. The small-diameter segment of the hollow bolt (1) is sleeved with a fastening nut (7). A correcting spring (3) is arranged between a spring seat (4) and the fuel capacity control pin (2). A starting enriching spring (5) is arranged outside the spring seat (4). The automatic fine-adjustment mechanism can automatically play a role in fine adjustment of the fuel pumping capacity of a diesel when the load of the diesel changes; in addition, automatic adjustment is good in reliability and small in fluctuation, the fuel consumption of the diesel can be lowered, and the service life of the diesel is guaranteed; moreover, the automatic fine-adjustment mechanism has the advantages of being ingenious in design, simple in structure, easy to assemble and reconstruct, low in reconstruction cost, high in universality and the like; the compression amount of the starting enrichment spring can be adjusted by adjusting the position of a screw to adapt to requirements of different situations, and adjustment is easy and convenient.