Nonreciprocal transmission device is one of the fundamental elements in integrated optics, and mid-infrared is a widely used waveband in many areas, such as remote sensing or spectrum analysis. An all-optical diode based on self-phase modulation (SPM) effect is numerically demonstrated in mid-infrared waveband. The diode consists of a linear waveguide and double silicon ring resonators. The nonreciprocal transmission ratio can be more than 20 dB in a power range between 0.5 mW and 20 mW, while the transmission loss in forward direction is less than 10 dB. Moreover, the influences of linear absorption coefficient of ring resonators and the bi-stability effect on the performance of the diode are discussed.