A non-volatile flexible-grid wavelength-selective switch (NVFGWSS) based on subwavelength-grating-Ge₂Sb₂Te₅ (GST)-assisted silicon microring resonators (MRRs) is proposed. By controlling the state of the subwavelength grating GST and the phase shifter, the transmission spectra of the designed subwavelength-grating-GST-assisted silicon MRRs are combined, and thus tunable bandwidths (BWs) are generated as required. A comprehensive analysis of the presented subwavelength-grating-GST-assisted silicon MRRs and the corresponding NVFGWSS is given. Numerical simulations reveal that, for the designed module comprising a subwavelength-grating-GST-assisted silicon MMR and an ellipse-based crossing waveguide, its maximum crosstalk (CT) and insertion loss are −18.08 and 0.50 dB, respectively. For the designed NVFGWSS, as the channel spacing is 0.8 nm, the in-band ripple and CT are <0.895 and −13.006 dB, respectively, and the 3-dB BW changes from 0.51 to 3.2 nm.