The invention discloses a control method for preventing the frequency of a pure DC networking power grid from being seriously deviated, which is characterized by comprising the step of emergently changing DC power of the pure DC networking power grid when a serious fault occurs through a DC system additional control method so as to prevent the frequency of the pure DC networking power grid from being seriously deviated after the serious fault and avoids a third defense line of the power grid from acting. Furthermore, the pure DC networking power grid refers that the power grid is connected with other power grids all through a DC transmission system, the DC system additional control method refers to frequency limitation control of direct current, and the operation of emergently changing the DC power for the pure DC networking power grid refers that the DC power is changed through frequency limitation control of the DC system. According to the invention, the power output of the DC transmission system is changed timely, and the frequency of the DC networking power grid is prevented from being seriously deviated. The control method disclosed by the invention has the characteristics of flexible operation control, ability of knowing normal operations of the power grid, economy and the like.