Charge weld greatly affects the rate of material utilization and the mechanical properties of extruded profile. The evolution mechanisms of charge weld during porthole die extrusion of ZK60 Mg alloy were studied in this study. It is revealed that the material flow velocity was a key factor determining the geometric shape and length of charge weld, and the new billet always firstly appeared in the zone with high flow velocity. The die structure and process parameters strongly affected the length of charge weld, relying on the adhesive effect between old billet and die cavity. The dense impurities and oxides distributed on the front of charge weld, and the partial dynamic recrystallization (DRX) took place in new billet zone. With the extension of charge weld, the density of impurities and oxides was reduced and the microstructure was refined. The old billet always owned high DRX degree, and its grain size increased with the extension of charge weld. The fracture of profile followed the order of uniform plastic deformation, strain concentration near charge weld and longitudinal weld, debonding of new and old billets close to charge weld, crack of peripheral old billet near charge weld, and complete fracture. In addition, the profile with charge weld experienced two-time stress drops during the tests due to the debonding of charge weld and the crack of peripheral old billet. The mechanical properties were deteriorated by the emergence of charge weld, while they were gradually improved with further evolution of charge weld.