Based on the theory of multiple avalanche domains, a two-dimensional numerical model for GaAs PCSS with opposed electrode structure is established. The influence of the width of the trigger region on the output characteristics of GaAs PCSS is investigated. Firstly, the switching transient of PCSS is analyzed. The results show that the rapid increase of the carrier concentration and the drastic evolution of the charge domain make PCSS operate in the ultrafast-switching mode. On this basis, this paper studies the influence of the width on the output characteristics of PCSS. The results show that the increase in the width can accelerate the rapid multiplication of carrier concentration and the rapid evolution of avalanche ionization domain, thus shorten the delay time and switching time of PCSS. Further more, the effects of different trigger positions on the delay time and switching time are analyzed. The results show that the delay time under cathode triggering is significantly lower than that under anode triggering, and the switching time is almost unaffected by the trigger position. The above conclusions can provide significant reference for the study on time jitter and synchronization of GaAs PCSS.