In the karst mountainous area, with the urban expansion, a large number of natural mountains have been embedded into the urban built-up area, forming the urban remnant natural mountains which have various ecosystem service functions. However, in the process of dense development within the urban area, these urban remnant mountains were often isolated and surrounded by the urban construction land, forming the ecological islands in the artificial interference field of urban built-up area. Construction of ecological network of the mountainous urban could be not only conducive to the protection of natural resources of urban remnant natural mountains, but also beneficial to enrich the service functions of urban green infrastructure ecosystem. Taking the downtown of Guiyang, a typical mountainous city in karst area, as research area, we comprehensively used morphologically spatial pattern analysis (MSPA) and landscape connectivity to identify ecological sources and determined the comprehensive resistance area based on the principal component analysis. We also identified and optimized the key connection corridors and nodes through the minimum cumulative resistance model (MCR), and constructed the ecological network at two scales of the administrative area and the built-up area of the research area. The results showed that:1) the overall distribution of ecological sources in the administrative region presented a pattern of "north and south facing each other", and the ecological sources in the built-up area were concentrated in the central part; 2) The high values of comprehensive ecological resistance were mainly concentrated in the middle of the study area and radially spread outwards, while the low values were mostly distributed in the periphery of the study area with large forest patches; 3) The number of essential, important and general corridors in the administrative region were 15, 21 and 69 respectively. The essential corridors were concentrated in the north of the study area, meanwhile, the number of essential, important and general corridors in the built-up area were 37, 113 and 227 respectively, and the essential corridors were mainly distributed in the middle of the built-up area; 4) There were 29 and 25 ecological nodes in the administrative and built-up areas, and 33 and 17 second-class ecological nodes, respectively. Finally, by superposition the ecological network elements of administrative region and built-up areas, the ecological network of the downtown of Guiyang City was constructed. This study could provide scientific and reasonable reference for coordinating ecological conservation and urban development in the central urban area of Guiyang city.