Central pose errors involving central position error and central orientation error are critical indexes for 6-degrees of freedom docking mechanism assembly quality. The performance of assembly could be evaluated during virtual assembly stage in terms of these indexes. However, the evaluation process for assembly has to employ Monte Carlo simulation, which leads to huge computation cost and usually makes the process impossible. This paper gives an insight of the relation between central pose errors and structural errors and finds that the central position error and central orientation error of docking mechanism conform to Rayleigh distribution model at symmetric pose configuration. In comparison, the maximum difference between results with Rayleigh model-based assembly performance and Monte Carlo simulation-based one is no more than 1.22%. With this model, the evaluation process for assembly performance could be facilitated significantly.