A device and method for adjusting a fuel supply advance angle of a multi-cylinder diesel engine includes a gear chamber, a connecting disc and a fuel pump, wherein the gear chamber and the connecting disc are in the diesel engine, and the fuel pump is at a left end surface outside the gear chamber. A main shaft end of the fuel pump extends into the gear chamber. A right end surface of the connecting disc connects to a timing gear. A right end surface of the timing gear has three penetrating circular-arc waist-shaped holes. Annular sizing blocks are arranged on the waist-shaped holes. A penetrating circular hole is at a tail end of the waist-shaped holes. The circular hole is provided with a T-shaped nut. A support screw is arranged in the T-shaped nut, and the screw head of the support screw is jacked against the annular sizing block.