To improve the interfacial strength between basalt fibers (BFs) and PA6,6, amino-silane coupling agents were applied to BF-reinforced polyamide 6,6 composites. The effects of their molecular structures, especially the number of amino groups and the corresponding chain lengths, on the mechanical properties of basalt fiber reinforced PA6,6 composites (BFRP) were then investigated. The results showed that even with the same type of silane coupling agent, the distinctive feature in its molecular structure is eventually displayed in the difference in the reinforcing mechanism to strengthen the interphase between BF and PA6,6, and consequently, its impact on the strength of the composites. The adhesive properties of BF to PA6,6 were promoted by the polar amino groups as well as the non-polar long CH2 chains of the silane coupling agents introduced to the BF surface. As the number of polar amino groups increased, the intermolecular hydrogen bonding with the -CONH groups in PA6,6 increased, thus leading to enhanced interfacial adhesion between BF and PA6,6. The interfacial adhesion was also improved by the physical cross-linking formed from the chain entanglement between the silane molecules and PA6,6 as the number of non-polar long CH2 chains of the silane coupling agents increased. The tensile strength of their composites showed similar interfacial shear strength tendencies between BF and PA6,6 in the order of long-chain-, tri-, di-, and mono-amino-silane-treated BFRPs. Long-chain amino-silane was shown to be the most effective in strengthening the interphase between BF and PA6,6. This is because long-chain amino-silane induces strong chemical and physical bonding with PA6,6 as it has a large amount of amino groups in the terminate part and long CH2 chains in the spacer part.