Abstract: The thermal-hydraulic system code RELAP5 was coupled with the TDOT-T 3D neutron kinetic code by parallel virtual machine (PVM). A parallel double-channel natural circulation boiling system was built by the coupled code and the instability boundary of the system was obtained. Two types of fuel with different geometries (flat-plate-type fuel and pin-type fuel) were used in the analysis respectively, and the influences of neutronic feedback on density wave instability were analyzed. For the flat-plate-type fuel system, it is found that the type 1 density wave oscillation (DWO) is suppressed by neutronic feedback significantly but the effect of neutronic feedback on type 2 DWO is weak. For the pin-type fuel system, the result shows that the effect of neutronic feedback on instability boundary of the system could be ignored.