The invention discloses a transmission line fault probability accurate evaluation method. Aiming at the condition that during the fault probability evaluation process, the instability position is indeterminate, the instability region of the fault is dynamically evaluated, the distribution of the instability region in different scenes can be obtained, and therefore the fault probability evaluation result is relatively accurate. Compared with a conventional bisection method, a conventional sectioning method and the like, the transmission line fault probability accurate evaluation method is advantaged in that the problem that after a rocking curve is obtained through numerical simulation, the evaluation of the system stability degree or the analysis of the system stabilizing mechanism is difficult is avoided, and the instability portion and the instability proportion of the line can be worked out accurately aiming at various working conditions. Through the result, the system instability probability can be worked out more accurately, and the possibility of that the difference between the calculating result when the line operates in special working conditions and the actual result is relatively large is reduced.