Abstract: Large-scale development of wind and solar resources in Gobi-desert-barren land areas of Northwest China presents significant challenges to the grid integration of wide-area renewable energy. Supplying flexible resources and realizing bundled delivery via a transit power grid is considered the most practical solution. Focusing on the long- and short-term fluctuations of wide-area wind and solar power and flexibility requirements at different scales, this paper investigates the optimal planning of long- and short-term energy storage. First, a cost-frequency characteristic model for long- and short-term energy storage is established to quantitatively characterize the performance of pumped storage and electrochemical storage and clarify their respective roles. Based on the spectral characteristics of the net load, a source-load spectrum decoupling method is adopted for analysis to distinguish high-frequency and low-frequency ranges and extract typical frequency components. Second, a scenario temporal reconstruction method following source-load spectrum decoupling is proposed. In addition to daily scenarios, a scenario cycle model dominated by low-frequency components is constructed. Third, a system model considering input, delivery cross-section, and transmission channel constraints is established. By incorporating the medium- and long-term demand curves and performance requirements of the receiving-end grid, a multi-timescale operation simulation model for the transit grid is developed, and further an optimal planning model for long- and short-term energy storage is formulated. A case study is conducted on an actual provincial-level system in Northwest China. The results demonstrate that the source-load spectrum decoupling method can effectively identify the core high- and low-frequency components within the net load time-series; long- and short-term energy storage complement each other and are indispensable for mitigating large-scale wind-solar fluctuations; compared with the direct outward delivery of wide-area wind and solar power, the bundled delivery approach via a transit grid yields significantly greater benefits.