Overall land use planning is important in land management. It is an urgent duty to draw up the planning for each level government. The land use structure is the most important part in county's overall land use planning. The optimization methods of land use structure contain usual method which is common and fixed quantity methods.This paper focused on filling up the shortage of the usual compiling of land use planning. In this paper we try to use mathematical model to determine the land use structure in fixed quantity to improve the planning. Gray linear programming is a dynamic linear programming. Its characteristic is that the parameters of gray linear programming are variant and the constraints are developing. It can make up for the shortage of linear programming. The parameters of constraints of gray linear programming are in gray region, we can plan either to use the upper limit or to use the lower limit or any value of the region. A group of parameters we choose corresponds to a determining plan, therefore the planning becomes adjustable. Using gray linear programming to optimize land use structure,several solutions of land use structure can be selected. The method has been used for Tianyang county's land use planning. In the planning, we choose a group of linear equations as the constraints of the system model. They represent the needing of economics balance and zoology balance, and the limits of labour force and financing.We use mathematical model to represent objective function of land use structure. In Tianyang county's land use structure model, we select 11 variables, they are:①cultivated land;②garden land;③forest land;④grassland;⑤land of town;⑥residential area in village;⑦land of independent industry and mine;⑧communication land;⑨land of irrigation works; ⑩ other water area ? barren land.We construct 13 kinds and 23 equations of constraints,they are:①totalland; ②total population;③macroscopical plan;④social requirement;⑤labour force;⑥water resources;⑦electricity resources;⑧finance;⑨ecological balance; ⑩ harmonious development; ? land supply; ? current situation and financial condition; ? mathematical model. Then we determine the gray parameters of the constraints according to different constitutions. Three plans of land use structure have been got, and the best one has been given. Comparing the land use structure of current situation with optimization solutions, especially optimizing area and planning area in the land use structure,we find garden land added mostly,which is coincident with Tianyang's social economic development plan. The maximization of objective function of land use structure is 2.85 billion yuan. This is accordant with Tianyang's 2010 social economic development goal.It is shown that the method is better than the usual one. The approach has been proved scientific and reasonable. It can be used in county's overall land use planning.