Accelerating the synergy between digitalization and green transformation is an intrinsic requirement for high-quality development and an important pathway for constructing a digital ecological civilization. A pressing question is whether digital economy policies are facilitating digitalization while simultaneously achieving regional green transformation. This study constructs evaluation indices for digitalization and green transformation at the provincial level in China. Based on a coupling and coordination model, the study calculates the digitalization-greenization synergism development index (DGSDI) for 30 provinces in China from 2012 to 2020, analyzing its variation characteristics and regional differences. Additionally, employing a quasi-natural experiment of national big data comprehensive pilot zones, the study empirically tests whether digital economy policies have achieved synergistic development in digitalization and green transformation and delves into their internal mechanisms and heterogeneity. The findings reveal: 1) Both digitalization and green transformation levels in provinces have improved annually, yet the overall levels remain low with clear regional disparities. The synergistic development level of digitalization and green transformation has continuously increased, yet it remains low overall. Only Beijing, Guangdong, Shanghai, and Jiangsu have a coupling coordination degree above 0.7, reaching a moderate coordination level, while 22 other provinces have a degree below 0.5, not reaching the coordination level.2) The establishment of big data pilot zones significantly promotes the levels of digitalization, green transformation, and their synergistic development.3) Mechanism testing shows that: firstly, big data pilot zones have promoted provincial-level digital development through two channels: digital industrialization and industrial digitization, promoting green development through green innovation. Secondly, there exists a circular and mutually reinforcing mechanism between digitalization and greening, which is the underlying logic of the coordinated development of digitalization and greening. Specifically, digitalization empowers greening by promoting green innovation and production through digital infrastructure construction, as well as the positive role of digital innovation capabilities in promoting green innovation; Greening driven digitization is achieved through the demand driven effect of green innovation on digital infrastructure and digital technology. Thirdly, big data pilot zones have strengthened the digital effect of green development, but has not strengthened the green effect of digital development. The reason is that big data pilot zones have improved green innovation capabilities, thereby stimulating demand for digital infrastructure and technology, and enhancing the traction effect of green development on digital development; However, due to the failure of digital industrialization and industrial digitization to promote the green transformation of production, big data pilot zones have failed to strengthen the empowering effect of digital development on green development.4) Heterogeneity analysis shows that in regions with stronger knowledge absorption capacity and higher levels of industrial structure upgrading, the construction of big data pilot zones has a more significant promoting effect on the collaborative development of digital and green development.