Abstract: The String structure consists of a rigid body at the top chord, a flexible tie rod at the bottom chord and a rigid strut between them, steel cable or steel tie rod is usually used as flexible tie rod, in the structural system, it is a tension member. Special shaped truss string structure is arranged at skylight of Wenzhou Airport, the design institute has put forward a clear allowable range for the internal force in the construction of tension string structure steel tie rod. According to the overall construction scheme of steel structure, passive prestressing technology was adopted in the stage of floor assembly of string structure, after the initial tension was applied, the whole roof was lifted to the design elevation. The standard value of internal force of steel tie rod at the end of construction was defined in the design, it was called the standard value of forming internal force. Taking the standard value of forming internal force as the target value, combined with steel structure construction scheme and steel tie rod tension scheme, the whole process simulation analysis technology was adopted, the standard value of initial tension of steel tie rod was tried to calculate. In order to ensure that the forming internal force of steel tie rod meets the design requirements, master the change process of internal force of steel tie rod, ensure that the structure is in a safe state, it is necessary to monitor the internal force of steel tie rod during construction. The internal force of steel tie rod measured by frequency method is directly calculated by the correlation between frequency and internal force. By analyzing the characteristics of the string structure of the skylight, it can be found that, both ends of the steel tie rod are connected by pins,the boundary conditions are approximate to ideal hinge, this kind of boundary condition is suitable for the vibration of steel tie rod, the disturbance to vibration amplitude attenuation and vibration frequency is minimum. Therefore, the frequency method is suitable for the internal force monitoring of steel tie rod in this project. Lifting process is the process of structural stress system transformation, it's a dynamic process, and also the most frequent process of internal force fluctuation of steel tie rod, it is necessary to monitor the internal force of steel tie rod. According to the features of the strain method, such as mature technology, strong adaptability to environmental changes, it is easy to operate, and reliable results. In the process of upgrading, the method of installing strain sensor on steel tie rod is adopted to monitor internal force of steel tie rod. Through construction simulation, election of measuring points, construction preparation and equipment installation, field test, data acquisition and analysis, the results and conclusions are as follows: 1)the results measured by frequency method and strain method are within the allowable range, the phenomenon of insufficient tension or over tension is avoided, the feasibility of the construction technology of passive prestressing is verified; 2)the variation trend of measured internal force is consistent with that of theoretical calculation, the validity of the measured values is illustrated; 3)the relative value and absolute value of the difference between the two measurement methods meet the requirements of the Code for Construction of Steel Structures GB 50755—2020, which shows that both methods are suitable for the project.