This paper proposes a technique for adopting Airy isostatic compensation model to calculate the 5ˊ×5ˊ average isostatic gravity anomaly. In calculation, 5ˊ×5ˊ average elevation was used and A～Q zone was divided into three areas, i. e., the near, the middle and the distant terrain. Columns of 1ˊ40" ×1ˊ40", 5ˊ×5ˊ and 15ˊ×15ˊ are used respectively for the near, the middle and the distant terrain, and forward formulae for rectangular column, mass line and particle are adopted to calculate topography-isostatic corrections. It is demonstrated from the isostatic gravity anomaly map of a certain area in South China that the isostatic gravity anomaly can eliminate the regional field caused by variation in thickness of the crust, and thus show prominently the local anomalous field.