Disclosed is a positioning device for a fence supplemented with a pet guardrail. The pet guardrail is fixed to a side of an existing fence in a side-attaching manner, and an upper crossbar and a lower crossbar of the pet guardrail are fixed to an outer side of the existing fence by bolts and covered and protected by front and rear cover plates, and both ends of the upper crossbar and lower crossbar are sheathed with a side sleeve for protection. In this way, the grid spacing of the existing fence can be reduced to prevent pets from escaping, and the overall appearance is aesthetic, and the overall structure avoids people or pets from being scratched by the bolts and just requires simple operations for the installation.