Mid-infrared tunable laser is widely used in several fields, such as medical, military, and environmental protection. Herein, a tunable parametric laser with a central wavelength of 2.6 μm is generated by pumping a KTP crystal based optical parametric oscillator (OPO). The OPO employs an extra-cavity single resonance structure. Type-II phase matching is used to obtain a maximum nonlinear coefficient. Experimental results demonstrate that the wavelength of the oscillator can be adjusted in the range of 2.4-2.8 μm. The maximum pulse energy is 12.6 mJ with a pumping energy of 155 mJ, whereas the conversion efficiency is 8.1%. The beam quality is measured to be five-fold of the diffraction limit.