Abstract: Building internal platforms has become an important approach for supplier enterprises to achieve digital transformation. However, the process of integrating external digital capabilities with internal business capabilities to form digital platform capabilities has not been fully revealed. By analyzing the longitudinal case studies of five leading supplier enterprises that have built internal platforms, this study reveals the construction path and formation mechanism of digital platform capabilities. The research findings are as follows: (1) Digital platform capabilities followed the construction path of "platform foundational capability - platform application capability - platform composite capability," demonstrating an advancing characteristic of "horizontal expansion - vertical enhancement"; (2) and form corresponding capabilities at each stage through three integration mechanisms of "transplantation - debugging - embedding"; (3) with the mechanism of "spatial-cross domain-structural" integration, the digital and business capabilities promote the formation and progression of the digital platform capabilities. The conclusions open the "black box" of digital platform capability formation, enrich and expand the relevant literature on capability construction and integration in platform contexts, and have guiding significance for manufacturing enterprises to accelerate digital transformation through building internal platforms.