Modal analysis of robots is the key to its structural dynamic design and fault diagnosis. The existing modal analysis methods based on experiment and numerical modal analysis still have some limitations, such as time-consuming, laborious and poor accuracy of calculation and analysis. In this paper, a macroscopic accurate linear elastic finite element modeling method of ABAQUS based on actual measured stiffness is proposed. And the typical posture of a six-axis tandem robot arm is taking as an example for modal analysis. Three modal analysis methods, including elastic finite element model of ABAQUS, multi-rigid-body model of ADAMS and experimental model analysis, are used to analyze the robot arm in detail to compare the advantages and disadvantages of its calculation, analysis, and application range. Moreover, the key factors affecting the accuracy of numerical modal analysis are analyzed. Firstly, the torsional stiffness of the main kinematic joints of the manipulator is measured directly by using the static measurement method for the first time in this paper. Then, the measured torsional stiffness value is converted into the corresponding elastic modulus value. And the corresponding elastic modulus value is input into ABAQUS to build a linear elastic model that can accurately reflect the actual stiffness of the robot arm macroscopically. Next, the measured torsional stiffness is taken as the stiffness of the torsional spring at the joint of the robotic manipulator and input into ADAMS for the multi-rigid-body modal analysis of the robotic manipulator. Finally, constrained modal tests are carried out for the robotic manipulator based on impact testing module in LMS Test Lab. And the experimental results are compared with the numerical modal analysis results of elastic and multi-rigid bodies. The results show that: 1) The measured torsional stiffness of the manipulator joint is a key factor affecting the accuracy of the numerical modal analysis. 2) The macroscopically accurate elastic finite element model based on the actual measurement can reflect the stiffness characteristics of the manipulator more faithfully. 3) The elastic finite element model constructed in this paper can give a high precision for the natural frequency values of several low-valued modes, while the multi-rigid-body model only has a roughly comparable precision for the natural frequency values of the first mode.