Abstract: Fiber-reinforced resin matrix composites (FRC) are extensively utilized in sectors such as aerospace, rail transportation, and wind energy due to their exceptional properties, including high specific strength, corrosion resistance, and cost-effectiveness. However, during the molding process, these composites undergo complex physical and chemical transformations that inevitably lead to the formation of internal defects. Notably, the rapid thermal cycling associated with additive manufacturing (AM) processes further escalates the likelihood of defect formation. Consequently, research into manufacturing defects and online monitoring is crucial for enhancing the quality of FRC produced through additive manufacturing. This study provides an in-depth summary of the mechanisms behind the formation of defects within the resin matrix of composites, including residual stress, porosity, resin-rich areas, and interfacial defects. It analyzes the impact of these defects on the macroscopic mechanical properties of the composites and elucidates the micro-level origins of each defect type. Subsequently, the paper consolidates online detection techniques based on Fiber Bragg Grating (FBG) sensors, strain and displacement sensors, acoustic emission technology, and thermodynamic monitoring. It discusses the technical characteristics, application scenarios, and limitations of these techniques, aiming to offer insights and references for scientific research in this field.