0D-1D mixed-dimensional van der Waals (MvdW) heterostructures have shown great potential in electronic/optoelectronic applications. However, addressing the interface barrier modulation and charge-transfer mechanisms remain challenging. Here, we develop an analytic model to illustrate the open-circuit voltage and charge-transfer state energy in PbSxSe1-x-quantum dots (QDs)/MoS2-nanotube (NT) 0D-1D MvdW heterostructures based on atomic-bond-relaxation approach, Marcus theory and modified-detailed balance principle. We find that the band alignment of PbSxSe1-x-QDs/MoS2-NT heterostructures undergoes a transition from type II to type I, and the threshold of size is around 5.6 nm for x=1, which makes the system suitable for various devices including photocatalytic device, light-emission device and solar cell under different sizes. Our results not only clarify the underlying mechanism of interfacial charge-transfer in the heterostructures, but also provide unique insight and new strategy for designing multifunctional and high-performance 0D-1D MvdW heterostructure devices.