Polylactic acid (PLA) filament 3D parts printed by fused deposition modeling (FDM) have poor mechanical properties because of weak fusion interfaces. This article shows that SiC-coated PLA filaments are effective means to increase mechanical performance of PLA composites that are microwave heated. Numerical calculations on temperature-rising characteristics and temperature distribution of the interface in the microwave field are shown. 3D-printed specimens of PLA/SiC composites were printed by FDM and heated in a microwave. The experiments show the SiC/PLA composite filaments have better temperature-rising characteristics and temperature distribution at 185 °C for 60 s in the microwave field, and this enabled the 3D-printed specimens to achieve in situ remelting on the interface and increased interface bonding between PLA filaments. The SiC/PLA composite specimens heated using microwave increased by 51% in tensile strength, 42% in tensile modulus, and 18.7% in interlayer breaking stress relative to PLA. These results provided a new approach for the improvement of FDM workpiece strength.