The nanocomposites that composed of hollow carbon nanoonion-like particles (HCNOs) decorated with Sn quantum dots (QDs) (Sn QDs/HCNOs nanocomposites) were directly prepared by long-pulse-width millisecond laser ablation of pure Sn metal target immersed in n-hexane solution under room conditions. Low-resolution and High-resolution transmission electronic microscope (TEM) images not only reveal that the as-obtained products are composed of two homogeneously mixed phases, Sn QDs and HCNOs, but also the tight bonding of Sn QDs with the HCNOs. Moreover, Sn QDs/HCNOs composites have uniform morphology and narrow size distribution. The HCNOs present continuous imperfect onion-like structure with hollow cores and have a diameter in the range of 80–100 nm. The Sn QDs with a diameter in the range of 3–5 nm are distributed uniformly on the HCNOs. This work indicates that the pulsed laser ablation in liquid could be applied to the direct synthesis of HCNOs from the decomposition of liquid media via ‘bottom-up’ approach and in situ decoration of them due to the formation of pure metal Sn QDs via ‘top-down’ strategy.