Nowadays, the cable use probability increases in major city, which may lead to lots of problems at low load such as surplus reactive power and terminal voltage increase because of its large capacitance to the earth. To solve this problem, this paper researches the effects of cable on reactive power of urban distribution network. It uses back/forward sweep method to do flow calculation based on the established model of distribution network for single cable line. On this basis, super position theorem is used to analyze the model of transformer substation with multiple cable lines. According to the results, this paper proposes the conclusion that the increasing of cable hardly cause surplus reactive power and terminal voltage increase. Meanwhile, configuring inductive reactive-load compensation equipments is also unnecessary, which provide references for projects.