In order to directly observe the electric field characteristics and study the charge structure in thunderstorms occurring in tropical regions, a balloon-borne strong electric field sounding is used to measure the vertical component of the electric field, temperature within the cloud and real-time location information of the sounding. Based on the principle of corona discharge, two 1-m-long metal probes are used as the sensors to detect the vertical electric field. In the summer of 2019, a result of electric field sounding within a local thunderstorm was obtained in the northeastern coastal area of Hainan Island, China. With the combination of an S-band weather radar, atmospheric electric field instrument and lightning locating network, the charge structure of the thunderstorm is analyzed in detail. The results show that the thunderstorm is a small-scaled local thunderstorm occurring in the afternoon, the sounding starting to be observed at the decay stage of the thunderstorm. In this period, lightning activities is rare, and the variation of ground electric field is similar to that of conventional summer thunderstorms. The whole sounding process lasts 34 min, during which the vertical airflow in the cloud is relatively stable, basically keeping 4–6 m/s. It can be seen from the electric field profile that the charge distribution in the thunderstorm cloud shows a complex charge structure which is composed of six charge regions. A negative charge region is lowermost, and above this the polarity alternates successively from bottom to up, where all charge regions are located above the melting-layer. Due to data interruption, it is impossible to accurately judge the upper boundary of the upper negative charge region and the information about the positive charge region above. The remaining charge regions are located in an altitude range of 6.0–6.3 km, 6.3–6.6 km, 6.9–7.3 km and 7.3–8.3 km, respectively. The charge densities in these four regions are –1.84 nC/m³, 1.80 nC/m³, –1.46 nC/m³, and 1.04 nC/m³, respectively. According to the existing data, the charge density of the uppermost negative charge area should be greater than –0.51 nC/m³. Moreover, the upper positive charge region (the fourth from bottom up) has the largest strength, followed by the negative charge region above it, both of which are more than 1 km in thickness. The electric field intensities in the other charge regions are relatively small. The pairs of positive and negative charge regions at the bottom are slightly different in strength and thickness.