The PASS Coupling adopts flat-mouth type design and it meets the requirement on passing with the gap between the coupling key shoulder and the pipeline. Under the prerequisite of guaranteed strength, the coupling with this type of structure is not necessary to complete clamp the pipeline and partial axial and radical shift are allowable. This coupling is suitable for flexibility-required pipelines. The design takes deviation of pipeline outer diameter into consideration and eliminates the disadvantageous factors, thus making fittings adaptable to pipeline deviation and their performance will be impervious.