The invention provides a farmland irrigation water amount calculation method and device. The method includes the steps that whether the water content of soil is lower than a preset water content or not is judged; if the water content of soil is lower than the preset water content, the required water amount of a target irrigation area is obtained; the effective precipitation amount of the target irrigation area is obtained; according to the required water amount of the target irrigation area and the effective precipitation amount of the target irrigation area in a time period, the irrigation amount of the target irrigation area is calculated. By means of the method and device, the problem that in the prior art, irrigation water consumption accuracy is low is solved, water requirement conditions of crops in the target irrigation area are quantitatively and periodically reflected, precise utilization of irrigation water is achieved, and unnecessary waste of water resources is avoided.