Mid-infrared 3~5 μm KTA optical parametric oscillator（KTA-OPO） is analyzed theoretically and studied experimentally,the factors impacting the conversion efficiency are analyzed deeply and optimized systematically.In our experiments,an external cavity and non-critically phase-matched（NCPM） KTA-OPO noncollinear 1.064 μm Nd∶YAG laser is employed.It is of an electro-optic Q-switched single-lamp double rod structure.With the repetition frequency of 1~25 Hz and the pump energy of 120 mJ,the idler output pulse energy as high as 17 mJ is obtained for the 3.475 μm wavelength,and the highest optical-optical conversion efficiency is up to 14.3%.