Based on the theory of activity-based costing and the idea of value chain, activity optimization and cost control are carried out on the whole construction process of agricultural water conservancy projects. Through the use of literature review, case analysis and statistical analysis and other methods, with agricultural water conservancy projects as the research object, and comparing activity-based costing with the traditional model of cost accounting, the writer carries out the corresponding value chain analysis, and comes to the following conclusion: the statistical calculation and analysis of the entire construction process based on activity-based costing can accurately and effectively control the costs of water conservancy projects. Through the use of activity-based costing, the cost data of agricultural water conservancy projects is more detailed and accurate, and cost management personnel can also find non-value-added activities, optimize the enterprise’s value chain and achieve cost control objectives through the analysis of the activity chain and value chain of agricultural water conservancy projects.