The authors define spacial-plane-overconstraint and theoretical-plane-overconstraint, and analyze sensitivity of the two kindsof overconstraints to errors of mechanisms with theoretical-plane-overconstraint. Ananalysis method of the overconstraints of the mechanisms is present, and the overconstraint numbers and its characteristic is obtained. The analysis results are useful of designing for self-adjustable mechanisms without overconstraint. With this understanding,a synthesis method of self-adjusting structures without overconstraintis given.