Two-gap capillary plasma generator is a realistic alteative to the conventional capillary in electrothermal lannchers. It is designed to accomplish two goals: repetitive operation and a compact power supply system in which capillary itself serves as a closing switch. A theoretical model is proposed to investigate the trigger process of this kind of generator. Both ablation case and no ablation case are considered. Trigger delay time is also theoretically analysed. The calculation shows that capillary wal ablation can greatly change the behaviour of trigger discharge arc and plasma flow generated. Trigger delay time can be drastically shortened.