Abstract: A novel sandwich core structure comprised of square carbon fiber reinforced composite (CFRP) square honeycomb and polymethacrylimide (PMI) foam fillers was proposed. Quasi-static compression tests on the novel core structures with different geometrical configurations were conducted. The results show that when the spacing between honeycomb walls was small, the failure mode of the square honeycomb was mainly brittle fracture. With the increase of the spacing between honeycomb walls, the stability of the core reduced and the square honeycomb presented a failure mode of buckling. The PMI foam fillers can significantly improve the compressive properties of the CFRP square honeycomb by increasing the compressive strength and specific energy absorption. With the increase of the foam density, both the compressive strength and specific energy absorption of the core show an increasing trend. The research can provide reference on the design of sandwich core structures for impact resistance.