Accurate estimation of the transmission power limit of the interval tie line is of great significance for guiding the safe operation of the interconnected power grid.Based on the deep study of Hopf bifurcation theory , An improved Hopf bifurcation index is proposed for evaluating the damping level of the system.The exponential and the state variables have good linear relationship,and easy to calculate and also can truly reflect the relationship between the state variables and the system damping ratio.Based on the random fluctuation data of the power of the interval tie line, The Subspace Identification Stochastic (SSI) identification algorithm is used to identify the state matrix of the active power of the contact line as the state variable. According to the proposed improved Hopf index to evaluate the running state of the contact line, the limit line estimation of the contact line transport under small disturbance stability constraints is realized The simulation results show that the proposed method can accurately estimate the transmission limit of the tie line, and has the advantages of simple calculation process and fast calculation speed, which is suitable for the on-line application.