The growth of China’s OFDI (outward foreign direct investment) is a unique feature of the “Belt and Road” project. Does China’s OFDI have a green halo effect on countries along the Belt and Road (B&R)? Is this green halo effect somehow asymmetrical? What is the underlying mechanism? This paper systematically examines how China’s OFDI exerts its influence on green technology spillovers, based on 56 B&R countries’ 2003–2019 panel data. This study makes three significant findings: Firstly, China’s OFDI has positive asymmetric characteristics in promoting green technology spillovers to host countries mentioned, which have lower income levels and openness. Secondly, strict relative environmental regulation can act as a “pressure pool,” significantly enhancing the “green halo effect”; Thirdly, China’s OFDI can help host countries obtain more green technology spillovers through three channels: expanding host countries’ economic scale, upgrading host countries’ industrial structure, and suppressing host countries’ use of non-renewable energy. These findings point the way for 56 host countries to better accessing green technology spillovers.