[Purposes] This study aims to establish macro and micro technical performance indicator systems for Trinidad Lake Asphalt (TLA)/styrene-butadiene-styrene (SBS) composite modified asphalt with varying TLA dosages, elucidate the mechanism of performance enhancement by TLA and SBS, and provide theoretical support for optimizing asphalt pavement materials. [Methods] Experimental analysis was employed, involving 13 groups of TLA/SBS composite modified asphalt with different mass ratios. The influence of TLA dosage on composite modified asphalt performance was investigated through performance tests (softening point, penetration index, maximum tensile force in force ductility tests) and microstructural characterization. [Findings] The results demonstrate that increasing TLA dosage significantly improves the high-temperature stability of the composite modified asphalt, with the softening point increasing by 0.221 °C per 1% TLA dosage increase, while reducing temperature sensitivity. The uniform distribution of ash phases enhances matrix stability through synergistic effects of surface roughness and pore structure. Based on the construction requirements of AC-10 asphalt mixtures, the optimal mass ratio is determined as TLA∶ SBS = 3∶ 7. Microstructural analysis reveals that pozzolanic ash phases strengthen interfacial bonding via a superposition effect, and the SiO₂-based mineral composition and unique particle size distribution of TLA significantly improve the toughness and tenacity of the composite modified asphalt mortar. The composite modified asphalt performance can be improved significantly through regulating the ash phase distribution. [Conclusions] The findings provide a method and a basis for material design and performance improvement of asphalt pavement for heavy-duty traffic in humid, hot regions.