For normal planar closed chain mechanism, it is based on ideal planar constraint condition. The authors took crank leader mechanism as an example, and analyzed influences resulted from uncertain factors, such as manufacturing error. The result shows it belongs to statically indetermination mechanism. After analyzing its over-constraint and eliminating it, in this way, it become statically determination mechanism and can be assembled without obstacle. However, in the process of practical running, the mechanism can give birth to new kinematic constraint, frictional constraint and dynamic constraint etc., so over constraint emerges again. Therefore, it is necessary to analyse its adaptability in total running process. The conclusion shows that design of adaptability mechanism normally eliminate geometry over constraint at first, then it is analyzed that whether there is new over constraints from all kind of external constraints in the process of running.