Aiming at the problem of real-time condition monitoring of wind turbine, a real-time monitoring method based on incremental relative entropy was proposed. Firstly, based on the analysis of the characteristics of sliding window data, the formula of incremental relative entropy which is suitable for real-time calculation was derived. And the time complexity of incremental relative entropy is O(1), which is lower than O(n) of conventional calculation method. Next, a real-time wind turbine condition monitoring method based on datadriven and normal behavior modeling was proposed and the incremental relative entropy was used as the index of real-time residual analysis. The effectiveness of the proposed method was verified by the actual gearbox fault data of a 2 MW wind turbine. The results show that relative entropy residual analysis can realize fault warning at least 8~ 10 days in advance, which is better than the conventional statistics. The calculation time of incremental relative entropy was only 0. 4% ~ 1. 9% of the conventional calculation method, which has significant advantages in real-time performance.