Abstract: While regionally the whole tectonic framework of the Yanshan intraplate orogenic belt is oriented in E-W and NNE directions,the tectonic line of the Lengkou fault strikes NW.On the basis of detailed field tracing of the Lengkou fault,combined with previous regional research results,the authors hold that the Lengkou fault resulted from polyphase tectonic activities and controlled Mesoproterozoic deposits.The fault activities since the Mesozoic may be divided into four phases:1.north-vergent thrusting in the early Middle Jurassic;2.normal-sinistral strike-slip movement in the terminal Middle Jurassic to Early Cretaceous;3.south-directed thrusting in the Late Cretaceous;and 4.dextral strike-slip movement in the late phase.