The invention provides a computing method for a transient stability limit of a transmission section of a power system. The method comprises the following steps of: S1, dividing a power grid into a power transmitting region and a power receiving region, increasing the output of the power transmitting region, reducing the output of the power receiving region, and increasing the transmission power of the section; S2, obtaining a transient stability limit, a limit operation mode and a control fault according to a set output changing scheme; S3, recognizing an instable cluster and a stable cluster of the control fault in the limit mode; S4, generating a new output changing scheme, increasing the output of a machine set in the instable cluster or starting a machine set which is not started, and reducing the output of the machine set in the stable cluster until the machine set stops; S5, obtaining a transient stability limit, a limit operation mode and a control fault according to the new output changing scheme; and S6, ending if the control fault in the step S5 is same as that in the step S2 and taking the transient stability limit obtained in the step S5 as a result, or otherwise, executing the step S3 to regenerate an output changing scheme and search a transient stability limit. The computing method has the advantages of high accuracy and good stability.