Under partially shaded conditions (PSC), a photovoltaic (PV) array gets more complex characteristics. Yet, it is very difficult and utterly necessary for PV system designers to understand and predict them. Other than the prevailing methodology of solving a set of non-linear equations, the proposed methodology focus on three key points of short circuit (0, I sc ), MPP( V mp , I mp ), and open circuit ( V oc , 0), and splits its V – I curve into three regions, which are replaced by three asymptote lines. From this new viewpoint, this study presents a novel Psim-based piece-wise linear macro-model and its circuit parameter values can be determined based on the three key points. Parameter estimation method is then developed to calculate the values of three key points under arbitrary irradiance and temperature by using only the datasheet under the standard test condition. The new model can be used to predict PV module characteristics for various operating conditions and understand the behaviours of a long PV string and a large-scale PV array under any PSC patterns on Psim platform. Compared with the published other models, the new model have merits of short running time, relative high accurateness, and reliable unique solution, avoiding the computational complexity and convergence issues. The accurateness of new model and formulas is experimentally validated and the computational error can meet the requirement of routine engineering analysis and design. Therefore, the new model and formulas will provide a powerful tool for one to learn the behaviour of a large-scale PV array in a complex scenario and optimise entire PV power plant, which involves DC/DC converter for MPPT and DC/AC inverter.