This　study　presents　a　comprehensive　geometric　modeling　and　error　propagation　analysis　for　a　3-degrees-of-freedom　spindle　head,　focusing　on　an　over-constrained　parallel　mechanism.　Four　geometric　error　models　are　established　for　the　over-constrained　spindle　head,　each　considering　different　combinations　of　constraint　wrenches.　A　set　of　sensitivity　indices　is　formulated　to　evaluate　the　effectiveness　of　these　models.　Comparative　sensitivity　analysis　suggests　that　a　model　incorporating　all　constraint　wrenches　is　suitable　for　error　propagation　analysis.　Two　error　propagation　indices　are　introduced　to　quantify　the　impact　of　the　geometric　source　errors　on　the　position　and　orientation　of　an　individual　limb　structure.　The　coupled　error　propagation　indices　account　for　the　kinematic　interactions　among　limb　structures,　allowing　for　a　detailed　analysis　of　the　spindle　head＇s　terminal　accuracy.　The　relative　deviations　of　error　propagation　indices　are　utilized　to　assess　the　quantitative　impact　of　kinematic　interactions　on　the　geometric　errors　of　an　individual　limb　structure.　Furthermore,　this　study　analyzes　the　cumulative　effects　of　kinematic　errors　from　three　limb　structures　to　reflect　the　influence　of　kinematic　interactions　on　the　terminal　accuracy　of　the　spindle　head.　The　findings　indicate　that　geometric　errors　of　the　limbs　in　such　an　over-constrained　spindle　head　are　mutually　compensatory,　reducing　linear　and　angular　errors　and　enhancing　the　spindle　head＇s　terminal　accuracy.