Higher and higher penetration of photovoltaic (PV) system considerably affects the voltage in power distribution systems and restricts the feed-in power of PV generation. To stabilize power distribution system and increase the profits of PV system owners, this paper proposes a dual-loop PV smart inverter (SI) capacity optimization method which collaborates with a distribution-level reactive-power transaction strategy. Due to the high dependency of optimal capacities of SI on their operations, the proposed dual-loop SI capacity optimization method is integrated with a two-stage voltage control strategy. The strategy optimizes the operations of on-load tap changer (OLTC), capacitor bank (C-bank), and reactive-power output of PV SI to minimize line loss and maintain the system voltage within an acceptable range. To verify the feasibility of the proposed PV SI capacity optimization method and voltage control strategy, the practical load and PV generation data are used in this paper. The simulation results show that the proposed method can effectively mitigate the overvoltage problems and increase the feed-in power of the PV system. Besides, the proposed distribution-level reactive-power transaction strategy can improve the benefits of both DSO and PV system owners.