Objective To design and implement a control algorithm in a 6 degree of freedom (DOF) robotic manipulator, so as to simulate the spinal motion and provide stable and efficient testing plan for biomechanical tests on spinal implants. Methods The recognition method of stiffness matrix for L2-5 spinal system was firstly studied for decoupling purpose. Secondly, the direct force control system under each axial motion was established by combining the 6-axis manipulator control system with the incremental proportion integration differentiation (PID) control algorithm. By using the 6-axis direct force control system, pure moment of 7.5 N·m was applied in the direction of main motion axis to simulate flexion-extension (FE), lateral bending (LB) and axial rotation (AR) motion of L2-5 spinal segment. Results The range of motion (ROM) of L2-5 segment in FE, LB and AR direction was 23.01°,27.92°,9.81°, respectively. A 7.5 N·m pure moment could be achieved in the main motion axis, while maintaining zero force/moment in the unconstrained axis with root mean square (RMS) errors being less than 3 N and 0.1 N·m, respectively. Conclusions The proposed algorithm of direct force control using PID controller with predetermined stiffness decoupling matrix was capable of applying pure moment to the spine under FE, LB, AR motion. The research findings have a relatively high value of engineering application for various biomechanical testing of lumbar vertebrae.