This paper aims to explore the rule of formation settlement induced by the construction of new tunnel obliquely passing underneath an existing tunnel，and to avoid the large settlement of the existing tunnel affecting the normal traffic operation. The deformation curve of normal strata caused by the excavation of the new tunnel is derived based on the modified formula for the width of settlement groove. Then，based on the equivalent layered principle，considering the effects of structure stiffness and the angle between two tunnels on the curve of settlement groove，the prediction formular is educed for the deformation curve of arbitrary strata caused by the excavation of the new tunnel under an existing tunnel. The reliability of the prediction formula is verified by the analysis of engineering examples. This study can provide a reference for similar cases.