Abstract: A servo tuning device adapted to a multi-axis machine tool at least having two linear axes and a rotation axis used for a moving base and a working platform to move relatively along the two linear axes and the rotation axis. The servo tuning device includes a reflection component, a photoelectric sensor and a processor. The reflection component is configured to be disposed on one of the moving base and the working platform and has a reflection surface. The photoelectric sensor has a light-emitting element and a light-receiving element facing the reflection surface. The photoelectric sensor is configured to be disposed on the other one of the moving base and the working platform. The processor records information of relative movement between the photoelectric sensor and the reflection surface for calculating a loop gain value used for tuning a servo setting of the two linear axes or the rotation axis.