As an important component of the terrestrial ecosystem, forests play an irreplaceable role in maintenance of biodiversity, carbon sequestration, water conservation, and timber supply. Based on the criteria for evaluating forest ecosystem services function (LY/T1721-2008), 12 forest types in Ji'nan City were selected as the research object, and the maintenance mechanism of the forest ecosystem services function in Ji'nan City was studied. It was concluded that the annual forest ecosystem in Ji'nan City has a value of 25.18 billion yuan for water conservation, soil conservation, carbon fixation and oxygen release, nutrient accumulation, air purification, and biodiversity conservation. Among these, water conservation, carbon fixation and oxygen release, and biodiversity conservation were ranked as the top 3. Populus nigra L. and Platycladus orientalis L. are dominant in the ecosystem services value of different stand types, whereas Paulownia fortunei, Larix spp. and Bamboo are less dominant. The value of the ecosystem services function of forests of different ages is mainly provided by middle-age and young forests. The maintenance mechanism of forest ecosystem services function in Ji'nan City is affected by many factors, including forest resource area, stand structure, forest origin, and nature reserve construction.