Abstract: The sidetracked horizontal well with ultra small radius has advantages for recover the remaining oil in sand edge or injection imbalance zones. The objective well sites are mainly on low production wells, shut-in wells, casing failure wells, and abandoned wells. But once the well is sidetracked, it could not be recovered. So it is necessary to study the threshold for layer and well selection of sidetracked horizontal wells with ultra small radius. Taking Duanxi pilot zone as an example, based on the numerical simulation orthogonal experiment plan, multiple regression method is taken to set up the correlations between the 10 years’ cumulative oil rate of the horizontal well with ultra small radius and the parameters such as effective thickness, dimensionless distance to the layer top, oil/water well spacing, average permeability, average residual oil saturation, penetrating completion degree of the horizontal well. Results show that the regression function accuracy is high, and satisfy the calculation need in field. Through the empirical correlation, together with breakeven analysis, the technology threshold for the main factors of layer selection is determined, therefore, the sidetracked layer of the horizontal well with ultra small radius can be optimized.