A fuel injection pump having a variable pressure chamber which can adjust a fuel injection timing according to each engine load with a simple structure and a lower manufacture cost compared to the conventional injection pump without a separate apparatus for moving a barrel as well as not modifying a shape of a plunger.The fuel injection pump having the variable pressure chamber further includes a pressure adjustment mechanism including a contact plug of which lower portion is inserted and installed in the upper cover; a cylinder of which lower portion is inserted and installed in the contact plug and having the variable pressure chamber formed inside thereof; a main fuel pressing hole formed in the upper cover and the barrel to connect the variable pressure chamber of the cylinder and a plunger insertion hole of the barrel; a piston installed in the variable pressure chamber of the cylinder to enable an upward and downward sliding motion thereof; and a manual pressure adjustment unit installed on an upper portion of the cylinder to enable a vertical height adjustment to elastically support the upper portion of the piston.