In engineering applications, the fracture of the thermowells caused by resonance becomes more and more an important issue. But most studies were limited to the resonance frequency and very few were concerned with the stress analysis at points of resonance or away from resonance for the thermowells. So far there is no comprehensive solution. Based on the specifications of ASME PTC19.3 TW—2010, this paper establishes relevant limits by analyzing the vibration frequency, and the steady-state static and dynamic stresses in the thermowells. A design method of the thermowells for engineering applications is proposed, and the specific procedures of the calculation and the evaluation are shown through an actual example. It is shown that this method can ensure the safety of the themowalls in engineering applications.