In this paper, we analyze the mono-pile wind turbine, comprising blade bodies, a hub, a cabin, a conical shell tower, a transition piece, and a mono-pile foundation. Our study objective was to determine the natural frequency of the mono-pile wind turbine above the mudline. We assumed the conical shell tower, transition piece, and mono-pile foundation to be a vertical steel tube (VST) with a uniform cross section and with the same elastic modulus. We used the lumped-mass method to simplify the mono-pile wind turbine above the mudline as two concentrated masses. One concentrated mass at the VST top is the summation of the mass of the blade bodies, hub, cabin and one-fourth the mass of the VST above the mudline. The other concentrated mass at the middle of the VST is one-half the mass of the VST above the mudline. Taking the mono-pile foundation below the mudline as a semi-infinite long pile, we used Zhang's method to obtain the horizontal stiffness and rotational stiffness of the pile top. In this way, we can determine the deflection of the VST under horizontal load at any point of the wind turbine above the mudline. The flexibility coefficient of the unit horizontal load at any position of the VST can be determined accordingly. Then, using both the flexibility and lumped-mass methods, we can determine the natural frequency of the mono-pile wind turbine above the mudline. Lastly, with an example, we retrieved the natural frequency value of the mono-pile wind turbine above the mudline. We then compared the analysis result with the mono-pile wind turbine natural frequency obtained without considering the interaction between the pile and elastic groundwork, and found little difference between them. The new theory we propose in this paper is more accurate theoretically and should be validated experimentally.