A few-cycle mid-infrared (MIR) laser is demonstrated via nonlinear self-compression in solid thin plates. In this novel solution, the anomalous material dispersion in the MIR band and the chirp induced by self-phase modulation are mutually compensated, which can achieve self-compression. Finally, with the 4 µm laser injection with 4.8 mJ/155 fs and few-cycle pulses with 3.44 mJ, 29.4 fs are generated with a high efficiency of 71.7%, and the system maintains very good spectral stability in 10 days. Compared with other post-compression methods, this self-compression technique has the advantages of high efficiency and robust and large energy expansion scale, which can be further extended to MIR lasers with other wavelengths and higher peak power.