The background and the meaning of studying on 3～4 μm KTiOAsO4 (KTA) optical parametric oscillator (OPO) were clarified. The researching status at present and the developing direction in the future of the KTA OPO were introduced too. The phase-matching mode, the temperature tuning characteristic and the structure of OPO cavity were calculated and theoretically analyzed. At the same time, some problems which should be considered, such as the best length of the KTA crystal, the effect of pumping power, the coating of the OPO cavity mirror, the structure and cavity length of the OPO, the mode matching between the pump source and OPO, were discussed. The designed 3～4 μm KTA OPO pocessed the repeating frequency of 1～40 Hz, the pulse-width less than 20 ns and the single pulse energy more than 30 mJ.