In this work, the toxicity of imidazolium-based ionic liquids (ILs) and the enhancement of high-solid anaerobic digestion by pretreatment were studied. Compared with [Bmim] Cl, [Bmim] OAc and [Bmim] BF4, [Bmim] PF6 had the highest toxicity. When the mass ratio of [Bmim] PF6 to grass was higher than 1: 10, biogas was not produced within 30 days. The ability to remove lignin and hemicellulose followed the sequence of [Bmim] OAc, [Bmim] Cl, [Bmim] BF4 and [Bmim] PF6. The crystallinity index of grass pretreated with [Bmim] OAc, [Bmim] Cl, [Bmim] BF4 and [Bmim] PF6 reduced by 73.83%, 54.44%, 17.52% and 7.47%, respectively. The pretreatment with ILs enhanced the methane yield of grass by reducing crystallinity and particle size. The grass pretreated with [Bmim] OAc had the highest methane yield, about 221 mL/g volatile solids, due to its good lignin removal ability and relative low toxicity. After 10 times recycling, the cumulative methane yield of grass pretreated by recycled [Bmim] OAc decreased by 11.95%.