External-soil spray seeding technology is a widely used method for ecological slope protection, playing a significant role in mitigating soil erosion, landslides, and other geological challenges. However, research on the technical stability of external-soil spray seeding is limited, resulting in suboptimal protective effects and hindering broader application. This review highlights emerging research themes for advancing ecological slope stability, including: ecological substrate, vegetated ecosystem, the mechanical properties and hydrological characteristics of the external-soil spray seeding technology. The review identifies new research themes for developing futuristic ecological slopes can be summarized as: (1) whether or not to find the stability models on the shear strength and bond strength of the substrate under water saturation, (2) how to establish models with the effect of grassland ratio, slope angle, seeding amount, and planting season on the long-term growth of ecological slope protection, (3) how to improve quantitative mechanical models between the roots and soil, (4) how to propose the relevant analytical and numerical methods for root-soil-atmosphere. The findings offer valuable guidelines for improving ecological slope stability and advancing the application of external-soil spray seeding technology.