Aiming at the positioning accuracy of the spindle of CNC machine tool, the method of detecting thermal deformation was studied, and a simple machining test was proposed to evaluate the thermal displacement of the machine tool in the axial direction (Z direction) of the tool. In this method, the change of geometric accuracy caused by the thermal effect of environment or machine tool was observed by repeatedly machining a machined specimen containing 16 cylindrical grooves. Taking a 5-axis vertical machining center as an example, the slotting test and the round hole milling test were compared. The results show that, the thermal displacement (negative Z direction) of the center point of the slotting test method is 19 μm, compared with the non-contact tool measuring system, the relative deviation is 18.8%; the thermal displacement results of this slotting test method are almost consistent with the hole milling test and the deviation is reduced, and it is suitable for vertical and horizontal combined machining centers with any size.