Solid sediment is a key problem in the industrial-scale production of acrylic acid by hydrocarboxylation of acetylene under high temperature and pressure. The activity and factors for the formation of sediments have been studied. Experiments demonstrate that 80% yield of acrylic acid was achieved, and the sediments were mainly caused by the concentration of Cu2+ under a certain amount of acetylene initial partial pressure. The two kinds of sediments have different particle size distribution and different particle morphology, but both contain carbon nanofibers. SEM-EDXS, Raman spectra, XRD, and elemental analysis indicate three types of coke (i,e, polymeric, filamentous, and graphitic) and three carbon species (i.e., amorphous carbon, carbide, and graphite). It is generally accepted that residence time, reaction temperature, and distribution of catalyst concentration have an impact on sediment formation and side reactions.