Abstract: Carbon fiber/flattened bamboo composite is a new kind of composite material to improve the application of bamboo in engineering products. The bonding interface is the bridge of the transfer force of composite materials, and the bonding property of the bonding interface is the key to the overall mechanical properties of composite materials. The effects of hydroxymethylated resorcinol (HMR) coupling agent on the bonding properties of carbon fiber/flattened bamboo composite were studied. The specimens were divided into four groups according to different forms and treatment methods of flattened bamboo surface. The vertical density distribution, strain distribution, stress transfer and microstructure of the bonding interface of carbon fiber/flattened bamboo composite were measured and analyzed. Results show that the bonding strength of carbon fiber/flattened bamboo composite after treatment with HMR coupling agent is increased by 42.7% compared with that of the untreated group. The bonding interface density of carbon fiber/flattened bamboo composite increases, the thickness of the bondline is wider and the strain distribution and stress transfer more evenly. HMR coupling agent plays a good role in bridging and acts synergistically with carbon fiber to make the stress transfer at the bonding interface more continuous and improves the bonding properties of carbon fiber/flattened bamboo composite.