The invention provides a control and protection system of a high-voltage DC transmission system. The control and protection system comprises telecommunication monitoring layer equipment, operating personnel control layer equipment, control and protection layer equipment and field layer equipment, wherein the telecommunication monitoring layer equipment is connected to a remote monitoring center through a telecontrol LAN network; the operating personnel control layer equipment carries out information exchange with the control protection layer equipment through an LAN network; the control protection layer equipment carries out information exchange with the field layer equipment through a field bus; and the field layer equipment achieves collection and control functions of a primary equipment state and system operation information. According to the control and protection system, reactive power control is achieved in the control and protection layer equipment, and reactive power control is completed through intercoordination with the whole control and protection system in DC station control equipment of the control and protection layer equipment, so that voltage of an AC bus of a converter station can be kept in a required range or the reactive power exchanged between the converter station and an AC system can be kept in a given range.