Abstract: Systematic analysis of the lateral cracking failure of P110 steel grade casing coupling in a horizontal well during acid fracturing was carried out.The causes of cracks were analyzed by methods such as macroscopic geometric size measurement, chemical composition analysis, mechanical property testing, structure analysis, scanning electron microscope observation and energy spectrum analysis.The results indicate that the stress concentration at the root of the coupling shoulder and the notch effect caused by the oversize inclusions induce cracks and cause them to expand, which ultimately leads to failure.The fracture has dimple topography, but the dimple is very shallow; there is no obvious sign of plastic deformation around the crack, which is a very dangerous form of fracture.