In order to achieve the target damage under the number of design load cycles, an on-line damage assessment system for fatigue loading test is designed based on the interconnection mode of distributed network bus. The system uses the dynamic signal acquisition instrument to collect the blade strain data, improves the Butterworth filter for filtering and smoothing, extracts the peak and valley values of blade load by the local maximum method, establishes the relationship between section load, cycle times and material Goodman curve, and calculates the maximum action coefficient of fatigue damage based on Markov frequency matrix method and fatigue cumulative damage theory. The fatigue damage evaluation system established for the fatigue test of a 56.5m blade is experimentally studied. The test results show that the fatigue loading damage assessment system can realize data testing and real-time processing, real-time assessment of blade damage degree in fatigue loading, and has the same accuracy as the data post-processing method.