Six-station measurements on electric field change of lightning flashes were carried out in the summer of 2002 in Qinghai area ［ (37°3.795′N, 101° 34.94′E), 2553.32 m above sea level］, the northeastern verge of the Tibetan Plateau. By using the point charge model, the height and magnitude of charge neutralized by return strokes have been fitted, with the nonlinear least-square method, based on the electric field changes of cloud-to-ground (CG) flashes observed in a typical thunderstorm with larger-than-usual lower positive charge center (LPCC) occurring in the afternoon of 4 August 2002. A total of 65 return strokes in 16 negative CG and 2 positive CG flashes has been analyzed. The results show that the height of charge region neutralized by negative CG flashes ranges from 3 km to 5 km above the ground. For the two positive CG flashes, the neutralized charge region is located in a height range from 5 km to 6 km. It shows that the thunderstorm has a triple charge structure. The average charges neutralized by a negative return stroke and a positive return stoke are about 1.48 C and 2.02 C, respectively.