The invention aims to provide a assembling structure of a writing tool, wherein the assembling structure is provided with a sealing module assembly of a pre-assembling module which is used for sealing, and the sealing module assembly is detachably attached to a main body of the writing tool, so that the production process can be carried out smoothly due to the fact that a assembling process with asealing part which is difficult to be assembled is performed in advance, when a defect occurs in each part, the defect part can be independently processed due to the fact that the separation structure of the main body and the sealing module assembly, so that the defect rate is reduced; the assembling structure of the writing tool comprises the main body, wherein a pressing portion, a pen tube andthe sealing module assembly, wherein the pressing portion is arranged inside the main body, and is operated by pushing and pressing; and the pen tube is inserted into the hollow interior of the mainbody, the sealing module assembly is detachably attached to the front end of the main body, is provided with a module which is installed inside the sealing module assembly, and the module is used forsealing and operating.