3D　molded　interconnect　device　(3D-MID)　is　a　kind　of　injection-molded　thermoplastic　part　with　integrated　electronic　circuit　traces.　Currently,　it　is　a　hotspot　of　the　electronic　and　telecommunication　equipment　industry.　Laser　direct　structuring　(LDS)　is　the　main　approach　to　fabricate　3D-MID.　Laser　scans　and　activates　the　surface　of　thermoplastic　parts.　After　plating,　the　activated　area　is　coated　with　copper.　In　this　study,　a　model　was　built　to　describe　the　mechanisms　of　interaction　between　a　substrate　and　laser.　The　nanosecond　laser　was　applied　in　the　LDS　process　to　manufacture　3D　circuit　on　3D-MID.　With　the　aid　of　variable　laser　pulse　width,　the　model　was　confirmed　by　a　series　of　experiments　including　investigations　of　roughness,　surface　structure　and　energy　spectrum.　Finally,　critical　factors　affecting　the　LDS　process　were　found　out.　They　are　effective　guides　for　many　LDS　applications.