In this paper, an elastic deformation modeling method of series robots with consideration of gravity that combines the finite element structure method (FESM) with the virtual joint method (VJM) is proposed to improve the positioning accuracy of robots. This method has characteristics of low computational complexity, high precision, as well as high real time. Compared with the previous research, the influence of joint and link mass on the elastic deformation of robots can be considered. Firstly, the entire robot model is split into several independent components and these components are expressed as a combination of a rigid body and a 6-Dofs virtual joint. Through the VJM, an extended kinematic model of series robots is built with these 6-Dofs virtual joints. Secondly, the stiffness parameters of robot components or virtual joints are extracted and considered comprehensively by the FESM. Thirdly, according to the extended kinematic model, the elastic deformation model is established to obtain the positioning error caused by external wrenches and gravity of robots. Lastly, a series robot SHIR5-II is taken as an illustration to perform elastic deformation modeling by the proposed method. Based on laser tracker and finite element software, the static compliance test, the static compliance simulation and the 7-Dofs VJM-based elastic deformation modeling of SHIR5-II robot are performed to verify the effectiveness of this modeling method. © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.