The invention relates to a variable-section engine body stitching bolt structure. The variable-section engine body stitching bolt structure comprises a bolt locking sleeve, a variable-section screw, an upper crankcase and a lower crankcase. The bolt locking sleeve is provided with an inner hexagonal nut, a polished shaft assembled with the upper crankcase and a threaded blind hole. The variable-section screw is provided with an upper outer thread, a lower outer thread, a small-diameter polished rod, a large-diameter polished rod, clamping faces, transition arcs and transition fillets. A variable-section through hole is formed in the upper crankcase, and a threaded blind hole is formed in the lower crankcase. The variable-section screw is tightly screwed and assembled on the lower crankcase through a thread. The upper crankcase is installed on the lower crankcase so that the variable-section screw can be put in the through hole of the upper crankcase. The bolt locking sleeve is assembled in the through hole of the upper crankcase and is tightly screwed with the variable-section screw rod through a thread. By means of the variable-section engine body stitching bolt structure, reliability of an engine body can be obviously improved on the premise that structural arrangement of the engine body is met.