The utility model relates to a fuel injection pump with a variable pressure chamber. The fuel injection pump with the variable pressure chamber is not provided with a device for moving a machine barrel and does not need to deform the shape of a plunger. Compared with an original fuel injection pump, the fuel injection pump with the variable pressure chamber is simple in structure and low in manufacturing cost. Fuel injection opportunity is adjusted according to loads of an engine. The fuel injection pump with the variable pressure chamber comprises a pressure regulation portion. The pressure regulation portion comprises a joint bolt, a cylinder, main fuel pressurization holes, a piston and a manual pressure regulation portion. The lower end of the joint bolt is arranged on an upper cover plate in inserting mode, the lower end of the cylinder is arranged on the joint bolt in inserting mode, the variable pressure chamber is formed in the cylinder, the main fuel pressurization holes are formed on the upper cover plate and the machine barrel so as to enable the variable pressure chamber of the cylinder and the plunger inserting hole of the machine barrel to be communicated with each other, the piston can be arranged in the variable pressure chamber of the cylinder in vertically sliding mode, and the manual pressure regulation portion with adjustably vertical height is arranged on the upper end portion of the cylinder and supports the upper end portion of the piston elastically.