Loop　circuit　incremental　method　is　applied　to　error　analysis　of　multi-loop　parallel　mechanism,　and　presents　the　loop　circuit　selection　principle.　Based　on　D-H　parameter　and　characteristics　of　mechanism　with　actuation　redundancy,　the　error　model　of　spherical　2-DOF　parallel　manipulator　with　actuation　redundancy　is　established　by　virtue　of　loop　circuit　incremental　method　which　includes　both　position　error　and　posture　error.　In　the　error　model,　mechanical　structural　parameter　error　is　considered,　at　the　same　time,　considering　the　deviation　between　axis　of　each　revolute　joint　and　center　of　sphere　from　the　reason　of　processing　and　assembling　etc.　These　make　the　error　model　more　close　to　actual　project.　Based　on　the　error　model,　the　reason　of　jammed　phenomenon　which　spherical　mechanism　appears　is　further　analyzed,　and　the　mechanical　error　is　estimated.　This　lays　a　theoretical　foundation　for　kinematics　calibration　research　of　the　prototype.