The precision of Earth’s gravitational field from GRACE up to degree and order 120 was studied for different inter-satellite ranges using the improved energy conservation principle. Our simulated result shows that; For long wavelength (L ⩽ 20) at degree 20, the cumulative geoid-height error gradually decreased with increasing range, from 0. 052cm for 110km to 1. 156 times and 1. 209 times as large for 220km and 330km, respectively. For medium-wavelength (100 ⩽ L ⩽ 120) at degree 120, the cumulative geoid-height error decreased from 13. 052cm for 110km, to 1. 327 times and 1. 970 times as large for the ranges of 220km and 330km, respectively; By adopting an optimal range of 220±50km, we can suppress considerably the loos of precision in the measurement of the Earth’s long-wavelength and medium-wavelength gravitational field.