With the growth of ecological civilization in China, a city in an ecozone takes the ecological protection as the main responsibility and affords the ecological security of the region and even the whole country. The landscape ecological security pattern of this kind of region is the basis for the protection of the social economy and the ecological environment in a coordination domain. This study selected Zhangjiakou, which is a typical ecozone city, as the research area. Based on source-sink theory, five major ecological functional areas with different protection key points were identified to construct the landscape ecological security pattern, while the key ecological protection areas, ecological restoration areas, and corridor system were located, which are the basis for ensuring urban ecological security. With these areas, the three-dimensional protection system, with a "point-line-polygon", were constructed. Additionally, the priority on the structure of ecological security pattern was put forward, which provides the basis for the construction of the ecological security pattern for Zhangjiakou. The results showed that the key protection areas for the landscape ecological security pattern of Zhangjiakou were missing, and the total area of the missing area was 1555.77 km², which accounted for 75.60% of the area of the ecological protection core. The proportion of the key ecological restoration area was larger, accounting for 24.27% of the whole area, and there were three main missing areas in the ecological corridor. In addition, to realize the supporting role of the regional ecology of Zhangjiakou, the ecological protection scope should be continuously expanded until the completion of the construction of the key area. This study provides an example of the construction of an ecological security pattern for an underdeveloped eco-city and the construction of an ecological environment protection system.