The diverse ecosystems in the mountainous regions of southwestern China provide various essential ecosystem services, which are critical for the ecological protection and sustainable development of the upper reaches of the Yangtze and Pearl Rivers. Although the scale dependency of ecosystem services has been demonstrated, the effects of different grid scales on the complex mountain ecosystem services remain unclear. This study selects Wangcao Town in Zunyi City, Guizhou Province of China, as a typical representative of mountainous areas. Utilizing GIS technology and the InVEST model, the response of five key ecosystem services to four grid scales (30 m, 100 m, 300 m, and 600 m) is analyzed. The results reveal that nutrient purification is significantly more sensitive to changes in grid scale compared to water yield, soil retention, carbon storage, and crop production. While increasing the grid scale alters the correlation coefficients between ecosystem services, it does not fundamentally change the trade-offs and synergies among them. Differences in ecosystem service responses and their relationships are observed across different slope gradients under varying grid scales. Most ecosystem services and their relationships exhibit greater sensitivity to grid scale in the 0-5º slope range compared to other slope gradients. The dominant landscape type plays a critical role in the scale dependency of ecosystem services in complex mountainous terrain as the grid scale changes.