The formation of avalanche charge domain in high-power GaAs device has been studied numerically. According to the numerical simulation results of the distribution of electric field in the bulk of the device, the variation rules of peak electric field within the domain and the domain width with the bias electric field, the carrier concentration, and the device length are obtained, respectively, and by the comparison of the variation rules with and without considering carrier impact ionization, the effect of the carrier impact ionization on the peak electric field and the domain width have been obtained. It shows that 1) the peak electric field is mainly determined by the bias electric field, the carrier concentration and the device length, and the domain width is mainly determined by the carrier concentration and the device length; 2) the carrier impact ionization and avalanche multiplication have a significant effect on the peak electric field and domain width; and 3) the value of the peak electric field and domain width are closely related to the position of domain in the direction of domain motion. Based on the variation of peak electric field and domain width, the formation of avalanche charge domain is discussed. Our results can deepen the understanding of breakover mechanism of GaAs devices and contribute to the optimization of GaAs devices performance.