The invention discloses a method for computing the transient stability of a system in control modes of a direct-current power transmission system in the technical field of security defence of power systems. The method includes determining equivalent faults of an alternating-current system and equivalent faults of the direct-current system; determining probabilities of occurrence of the various equivalent faults according to statistic analysis for scheduling and running; selecting the control modes of the direct-current power transmission system; sampling the various equivalent faults by the aid of a uniform-distribution and normal-distribution pseudo random number generator; and computing stability parameters of the alternating-current system and voltage parameters of the direct-current system by the aid of an analysis software package of the power system, and computing the system transient stability in the control modes of the direct-current power transmission system. The method has the advantage that degrees of the influence of the control modes of the direct-current power transmission system to the transient stability of the alternating-current and direct-current hybrid system can be accurately evaluated.