Abstract: Current pre-drilling geosteering models are often built through fine seismic data processing, with low resolution. So, it is necessary to develop a high accuracy method to predict a reservoir interface. Based on logging data, a calculation method of apparent formation dip was proposed. A reservoir boundary model was constructed and the mathematical relationship of reservoir boundary was deduced by means of translation and virtual wells to determine the reservoir information on horizontal well direction. The prediction model of apparent formation dip and the mathematical model of reservoir boundary under different location distributions have been used in oil fields. The results showed that the apparent formation dip of target stratum predicted on site was 0.70°, and the apparent formation dip calculated upon well completion was 0.53°, indicating a minor error. The predicted result with the mathematical model of reservoir boundary had an error of 0.1-3.6 m with the measured result, which was significantly affected by formation changes. The research suggested that the depth of formation boundary was a function of horizontal displacement. For the downdip formation, there was a linear positive correlation between the boundary depth and the horizontal displacement. Further, there was a linear negative correlation between the boundary depth and the distance from modeled well. For the updip formation, the changes were opposite. For the formation in which thickness changed gradually, the mathematical model provided more accurate prediction results than the apparent thickness method. The reservoir interface prediction results could provide direct and quantitative guidance for horizontal well geosteering.