A high-efficiency 2 μm optical parametric oscillator based on MgO-doped periodically poled LiNbO3 intracavity pumped by a 1.064 μm Q-switched Nd:YVO4 laser is reported. With the intense fluence inside the laser cavity, a maximum 2 μm average power of 3.5 W is obtained at 15 kHz repetition rate in a degenerate state. A maximum optical-to-optical conversion efficiency of 17.5% with a slope efficiency of 25% is achieved when the laser diode power is 20 W. To the best of our knowledge, the efficiency is the highest ever achieved from an intracavity degenerate optical parametric oscillator in 2 μm region. The pulse duration of 2 μm is 1.4 ns, which is much shorter than that of 1.064 μm. A bandwidth of 30 nm is obtained at a degenerate wavelength of 2128 nm. The M2 values for the 2 μm beam are 3.47 and 3.54 in the horizontal and vertical directions, respectively. The standard deviation of the power fluctuation is ～ 2% at the maximum power in half an hour.