Photovoltaic (PV) DC collection systems offer advantages such as high efficiency, low-cost, and good stability. The multi-port, modular DC collection systems have strong controllability and ease of voltage boosting. However, power mismatches among the power generation units often lead to module output voltage imbalances, thereby restricting generation efficiency and system stability. To address this issue, a two-stage boost DC collection scheme is proposed based on PV-storage coordinated voltage self-balancing technology. First, an input-independent-output-series (IIOS) PV DC collection system is constructed based on heterogeneous sub-module integration. This system achieves voltage boosting and collection of independent PV units while ensuring output voltage self-balancing among modules. Then, a second-stage boost is realized using an isolated MMC DC-DC converter, reducing the boosting requirement at each stage. Finally, a simulation model is built in the Matlab/Simulink environment. Simulation results show that the proposed PV DC collection scheme exhibits high reliability and efficiency, maintaining excellent power generation performance and system stability even under severe power mismatches among generation units.