The invention discloses a low excitation limit medium and long-term model building method and system based on digital-analog mixed real-time simulation. The method comprises the following steps: establishing communication with an excitation regulation controller and a generator set and receiving a measurement signal; Setting an initial low excitation limit parameter according to a preset rule, wherein the low excitation limit parameter is determined according to a system static stability limit and a thermal stability limit; Adjusting the low excitation limiting parameter in real time accordingto the change of the terminal voltage signal; Obtaining the basic parameters of the medium-long term model according to the preset method, and establishing the medium-long term model of the low excitation limit according to the basic parameters of the medium-long term model and the low excitation limit parameters; The method and the system build a digital-analog mixed simulation model to more truly reflect the actual system characteristics, and the low excitation limit parameters are set and adjusted in real time according to the voltage signal change of the generator terminal to ensure the excitation level of the generator unit to be stable.