Abstract: The shale oil and gas reservoirs in China are deeply buried, where an optimal and fast drilling cannot be realized by following the traditional five-section three-dimensional (3D) horizontal well trajectory. In this regard, a seven-section (i.e. vertical section, build section, hold section, direction turning section, hold section, build section and horizontal section) 3D horizontal well trajectory was proposed, together with the corresponding basic equations (ternary nonlinear equations). The seven-section 3D horizontal well trajectory was split into two vertical planes and one inclined plane, and then three equivalent binary nonlinear equations that are more simple to solve were developed. The case study shows that the application of this seven-section trajectory to 3D cluster horizontal wells in deep reservoirs can greatly reduce the length of the 3D direction turning section, which is conducive to ensure the well trajectory control and improve the drilling speed. The findings of this research provide technical support for the design of 3D cluster horizontal well trajectory.