Abstract: In order to solve the rotor winding in-ter-turn short circuit in a turbine generator, an online diagnosis method based on multivariate state estimation technique（MSET） was proposed. The state matrix was built by using sliding window and Euclidean distance. The exciting current and reactive power calculation models were constructed based on MSET. According to the average value of the relative deviation or residual value between the operating value and the calculated value of the generator, an online diagnosis method for rotor winding in-ter-turn short circuit fault was constructed, and the operation data of a steam turbine generator was used for verification. Results show that, the proposed online diagnosis method is feasible, which meets the requirements of rotor winding in-ter-turn short circuit fault diagnosis, and reduces the probability of fault misjudgment.