Against the backdrop of data spaces becoming the core of global data strategy, constructing an efficient and trustworthy government data sharing mechanism is a critical proposition for national digital transformation. This paper focuses on the European Union's "Once-Only Technical System" (OOTS), viewing it as a key practice of the data space concept in the government domain. The aim is to analyze its operational mechanism in the context of data spaces. Based on Activity Theory, this paper constructs a three-dimensional framework of "Institutional Design-Technical Architecture-Ecological Synergy". This framework reveals how OOTS implements the core principles advocated by data spaces-such as data sovereignty, mutual trust and interoperability, trusted connections, and value co-creation-through a complete legal system and givernance system, a standardized set of interoperable technical tools, and a collaborative network of diverse stakeholders. The study finds that the successful practice of OOTS provides a systematic data space solution for overcoming the limitations of traditional government data sharing and establishing cross-domain trust. Accordingly, this paper proposes strategies for optimizing the government data sharing mechanism under China's data space strategy from three dimensions: improving top-level institutional design, constructing an interoperable technical architecture, and cultivating a collaborative governance ecosystem.