Abstract: The Erlangping back- arc basin was an important part of the North Qinling active continental margin in early Palaeozoic Era. The Erlangplng Group is the remains of back- arc basin, and records its evolution. Analysis of the sedimentary textures demonstrated that the Southern and Northern clastic sedimentary rocks were formed in different active continental margin. Besides, there are two series of subductional granites which distributing along the two boundaries of Erlangping back are basin. Tectonicly, the main orogenic period of deformation can be derided into an early stage and a late stage. Two sets of thrust ductile deformation were formed in early stage while the complex tight folds were formed in late stage. The characteristics of sedimentation, magmatism and deformation all indicate that this back- arc basin is characterized by bidirectional subduction during late Paleozoic Era, i.e., a subduction downward to the Kuanping ancient continent, and another downward to the Qinling ancient island arc.