The wind turbine blade usually suffers from fatigue fracture occurred easily under alternating load. So health monitoring of wind turbine blade fatigue damage is very important. A new study method of wind turbine blade fatigue damage is proposed based on thermodynamic entropy. The changing law of the cumulative entropy production of the blade vs. the number of fatigue cycles is analyzed and the threshold point of fatigue fracture occurrence is determined. The results show that the damage energy of the wind turbine blade is also an important factor affecting fatigue damage. There are three stages for the cumulative entropy production of the blade vs. number of fatigue cycles. Study reveals that the threshold point of fatigue damage is the starting point of the third stage of the cumulative entropy production curve. The cumulative entropy production at threshold point and fatigue fracture entropy are independent fixed values, which are not affected by load, frequency and stress ratio. Through calculation, it is found that the ratio of cumulative entropy production to fatigue fracture entropy is 05. Other fatigue experiments were used to verify the threshold point, the result proves that the determined threshold point of fatigue fracture is accurate.