High proportion of distributed photovoltaic power generation (PV) generally exists in the distribution network (DN). However, high PV penetration leads to power factor reduction, power quality decline, line loss changes, and other problems. Generally, most indexes in the power system are calculated according to the parameter conditions of the main network (MN), which means X>>R is set by default. However, it is different between the line parameters of the MN and the DN, causing significant errors in the calculation of the DN using the MN conditions (X>>R). This paper examines the causes and processes of the changes in the critical indexes caused by the incorporation of a high proportion of PV into the DN, such as grid-connected voltage and line loss, based on the different line parameter conditions between the MN and the DN. Considering the DN condition (X>>R is not tenable), a novel calculation method of main operating indexes for the DN with high PV proportion is proposed. Finally, a numerical model built on an actual DN is used to verify the conclusion by comparing the difference in the change process of the main indexes generated by the calculation based on the MN condition and the novel calculation method in the event of high PV penetration in the DN.