Adhesive joints offer advantages over traditional joining methods due to their lightweight nature, reduced stress concentration, and ease of manufacturing. Their mechanical performance is influenced by various factors, including defects, which can significantly affect the performance of the joints. However, research focusing on fracture mechanisms of adhesive joints influenced by defects at microscale is still limited. This study conducts both experimental and numerical investigations into the effect of microstructural defects on the performance and fracture mechanism of multi-type adhesive single lap joints (SLJ). The adherend materials are aluminium alloy (Al) and polyphthalamide (PPA), bonded with an epoxy adhesive. Mechanical properties of the adhesive, adherends and SLJs, obtained through experimental studies, are employed to calibrate the microparameters in Discrete Element Method (DEM) models for numerical analysis. The developed DEM models can predict the performance and capture the microstructural fracture mechanisms of multi-type SLJs, through realistically incorporating different types of microstructural defects, including the interfacial and adhesive defects. Finally, the influencing mechanisms of microstructural defects on the performance and fracture mechanisms of multi-type SLJs with different interfacial adhesion are investigated, including joint strength, microscale crack initiation, coalescence, and propagation.