The urban forest is a vital carbon sink base in a city. The carbon-oxygen balance capacity of urban forests affects the urban carbon cycle and urban sustainable development. The forests maintain the carbon-oxygen balance through carbon sequestration and oxygen release (CSOR) processes. The carbon-oxygen balance of urban forests is formed by offsetting the carbon release and oxygen consumption (CROC) process of urban social activities through the CSOR process of forestland. Based on GIS technology, this research used the carbon-oxygen balance model to analyze the CROC and CSOR and study the carbon-oxygen balance of urban forests in the southern mountainous area of Jinan, China. The results of the increase in the carbon-oxygen balance coefficients showed that the carbon-oxygen balance capacity of urban forests showed a decreasing trend, with the decrease in forest area and the increase in fossil energy consumption from 2000 to 2019 in the southern mountainous area of Jinan. To increase the urban carbon-oxygen balance capacity, the city should expand its woodland area to improve the urban forest’s CSOR capacity and adjust the urban energy consumption structure to reduce the CROC of urban social activities.