This study focuses on the issue of hot tearing that is easily encountered during the semi-continuous casting of 6101 aluminium alloys. Flat ingots with hot tearing were characterized using SEM and other analytical techniques to clarify the origin and propagation mechanisms of hot tearing. The results reveal that the hot tearing in the 6101 aluminium alloy manifests as longitudinal cracks, which initiate from the agglomeration of boride inclusions formed by impurities such as V, Cr, Mn, and Ti. These borides originate both from residual boride inclusions after the boron treatment process and from newly formed borides resulting from reactions between impurities remaining in the melt and the Al-5Ti-1B grain refiner, which are transported into the mold during casting and subsequently solidified within the ingot. Combined with ProCAST numerical simulations of stress distribution during semi-continuous casting, significant temperature gradients between the ingot core and surface, reduced solidified shell thickness, and localized stress concentration lead to crack initiation in regions rich in inclusions, thereby promoting the formation of longitudinal cracks. On the basis of these findings, an optimized measure is proposed: prolonging the holding time after boron treatment to eliminate large agglomerated inclusions, reduce the susceptibility to hot tearing, and simultaneously meet the requirements for high electrical conductivity.