Abstract: Well instability of laminae shale oil with weak surface development has been a worldwide technical problem in drilling engineering, as well as one of the key issues in safe and efficient drilling. The wellbore collapse of horizontal wells in shale oil has caused great difficulties to drilling engineering, which has triggered multiple engineering accidents and complicated situations. Wellbore instability accidents prolongs the drilling cycle and greatly restricts the efficient development of shale oil horizontal wells. In this paper, core displacement experiments and nuclear magnetic scanning are carried out to determine the depth of drilling fluid penetrates into laminae shale oil under different displacement time. Based on the experimental results, a new mathematical model of drilling fluid penetrate laminae shale oil is established and brought into the numerical model for verification. The penetrate depth of drilling fluid along the laminated shale oil has a positive logarithmic relationship with the penetrate time. With the increase of the penetrate time and the laminar opening, the invasion depth gradually increases, but it increases slowly in the later period and eventually tends to remain unchanged. With the increase of the number of laminae, the invasion depth decreased gradually. This study aims to reveal the invasion mechanism of laminae shale oil under drilling fluid invasion and provide theoretical basis for the selection of safe drilling fluid density and wellbore instability.