Porosity is one of the most unavoidable defects in composite part which has significant impact on structural strength. The sandwich structural composite parts were manufactured by vacuum bag-autoclave co-curing method. Porosity of sandwich composite part laminated area with variable thickness was studied by adjusting compaction parameters including compaction vacuum, compaction time and ancillary material types. Results show that improving compaction vacuum, extending compaction time appropriately and using perforated release film A4000RP3 and breather Style PC3 can reduce the porosity of composite part variable thickness area.