To construct the model of "Ecology-Agriculture-Construction Comprehensive Planning" for delimitation techniques of urban development boundary, this paper calculates the proposed scale of county construction and development under spatial constraints based on "Short Board Theory", evaluates the suitability of land space development by calculating the comprehensive score according to the index weight, determines the land plot planning function and identifies construction priority areas in accordance with the principle of ecological priority and cultivated land protection priority. Then on these bases, the urban development boundary shall be delimited according to the urban construction conditions, laws and regulations, social and economic development, urban and rural coordinated development and other factors, with the priority areas as the base map and the scale of urban construction land as the basis. Through the empirical research on the central area of Xiangtan county, the urban development boundary of 4500 hectares has been determined. Compared with the scale boundary of the land-use master plan and the spatial growth boundary of the central district of the urban master plan, the technical model fully considers the restrictive effects of ecological environment, land resources, water resources, natural disasters and other factors on construction and development. The ecologically important or sensitive areas in the urban development boundary have been reduced from 3.49%, 12.46% to 0. The proportion of cultivated land occupied by new construction has been reduced from 53.26%, 50.94% to 21.02%, without occupying permanent basic farmland and ecological forests. The average annual growth rate of new construction land has been reduced by 45.65%, which is divided into or radiated into the concentrated area of village distribution. There are some problems in the traditional methods, such as determining the scale of construction land in the light of the development needs without considering the constraints of resources and environment on urban development, and ignoring the multiple suitability of land, etc. The research result shows that the technical model proposed in this paper makes up for the above shortcomings, so as to realize the priority of ecology and farmland protection, and promote conservation and intensive land use and urban-rural coordinated development. In addition, it can play a positive role in optimizing the layout of land space development, utilization and protection, and boosting green and high-quality development.