As one of the significant measures to realize energy transition, the proportion of new energy in energy supply side becomes increasingly high. However, the fluctuation and randomness of new energy output bring great challenges to the conventional generation expansion planning (GEP). Under the framework of combining investment decision and operation analysis, a novel GEP method for integrating large-scale new energy generation was proposed, which mainly includes: adopting a monthly time resolution in investment decision-making stage to consider the seasonal fluctuation and rapid construction characteristic of new energy; adding an ultra-short-term operation model to operation analysis stage, which can be used to evaluate the system adjustment capacity during typical periods with sharp changes of net load (such as morning peak period and evening sunset period) and reflect the value of flexible resources in line with the operation of new energy; furthermore, through the coordinated iteration and rolling correction between investment decision and multi-time scale operation assessment, the optimized planning scheme with comprehensive benefits of economy, environment, reliability and flexibility was obtained. Finally, to provide a reference for future research, the key problems that can be further studied are discussed.