It is crucial to determine the evolution and distribution of the base pressure of wind turbine spread in designing the large-scale and large eccentric shallow foundations and ensuring the safe and stable operation of wind turbines in the mountain areas. Based on the measured data of the base pressure of wind turbine spread subgrade on an inland residual soil, the long-term and short-term changes and distributions of the base pressure are analyzed. A concept of the flatness of measured base pressure is defined according to the testing results, and the formulation from the building code is modified according to the calculated flatness of measured base pressure. It is shown that the values and distribution of the base pressure have a close relationship with wind speed and operating conditions, and the distribution of the base pressure can be represented by a trapezoid or a parabola. It is also shown that the variations of the base pressure with rotor rotation, cabin vibration or wind randomness should not be completely ignored. There is a disparity between the distribution of the measured base pressure and the ideal plane distribution. When the current code is used to calculate the maximum and minimum of wind turbine base pressure, correction factors of 1.2 and 0.8, respectively, are suggested.