In the sending system of wind-photovoltaic-thermal-bundling power grid transmitted by multi direct current(DC), the new energy and DC transmission proportion is large, system inertia level is low, carrying capacity of alternative current(AC) grid structure is not strong and stability issues of power angle, frequency and voltage are prominent. Based on summarizing new characteristics of the northwest China power grid after wind-photovoltaic-thermal bundled, problems during security operation of the northwest China power grid are analyzed, and shortcomings of existing perceiving and controlling methods are pointed out. The security and stability defense system construction scheme of wind-photovoltaic-thermal bundling power grid for DC transmission is focused. Besides, main functions of defense system are elaborated in detail, such as synchronous stability controlling, frequency controlling, full process voltage controlling and key technologies of major breakthrough. The evaluation effect shows that the defense system can effectively deal with fault impact of ultra high voltage direct current, block cascading faults, and improve security and stability level of the northwest China power grid operation. The technical scheme guides the development of controlling technology and engineering practice for power grid with wind-photovoltaic-thermal power bundling and multi-DC weak sending-end.