Joint fire attack is the most important operation modes in modern wars. Based on the analysis of the joint fire attack's circle process, the optimal time cycle of target assignment is put forward. According to the states of both sides and the battle situation updating, a thinking mechanism for target assignment is established. Connecting with the input variable and the constraint expression, a linear programming model is made up through separating the battlefield into girds to reduce the complexity of target assignment. At the end, an example is given to verify the model. The use of this model can optimize the weapon target assignment, which provides a quantitative analysis tool for setting down the campaign scheme.