Abstract: Two-layer fracturing with original production string is a novel technology to implement environmental protection operation and improve labor efficiency. This paper analyzes the force of downhole strings and various fracturing tools used by this technology, including buckling, deformation, axial force and stress and strength. Wellhead and top section of packer are believed to be the dangerous section of strings. Then application software is compiled. After inputting a series of parameters, we can get the internal pressure and external pressure, axial force and stress intensity, safety factor and the state of axial deformation at wellhead and downhole packer. Inputting parameters include well trajectory, casing program, tubing combination, friction coefficient, fluid property in tubing, temperature, injection mode, tubing pressure at wellhead and the anchoring state. This technology has been applied in actual wells to guide the design of fracturing strings and the selection of tools effectively, thus ensuring the success of the fracturing operation.