In order to understand the synergistic effects between dielectric barrier discharge (DBD) plasmas and catalyst in the one-stage plasma-over-catalyst (POC) system, effect of DBD plasmas on C2H4 in the gas phase and adsorbed on the CuZSM-5 surface has been studied by adsorption, temperature programmed desorption (TPD), temperature programmed oxidation (TPO) and emission spectral techniques. The results demonstrated that the species adsorbed on the CuZSM-5 surface (such as CO2 and H2O) could be partly desorbed by DBD plasmas, and in the meantime the new species could also be formed by plasma-induced surface reaction. The structure of the carbonaceous deposit over the CuZSM-5 was changed and the “light-off” temperature for coke direct combustion reaction lowered by plasma-over-catalyst-induced processes. Some short-lived active species could be formed from plasma-induced fragmentation of C2H4. NOx removal was tested by DBD plasmas or a POC reactor. Without addition of C2H4, NOx conversion percentage was only 8% by pure plasma-induced processes in NO/O2/N2 system. With addition of C2H4, the NOx conversion percentage was increased to 18%, and large quantities of NO2 and CO were generated. The NOx conversion percentage could be up to 79% using the POC reactor in C2H4/NO/O2/N2 system. The formation of some stable species and short-lived active species from plasma- and plasma-catalyst-induced processes may be responsible to the observed synergistic effects between the plasmas and catalysts in the one-stage POC system.