Different from classic polyelectrolytes, poly(ionic liquid)s (PILs) bearing hydrophobic counterions show high electrorheological (ER) effect in dry state. This opens a way to develop new generation of anhydrous polyelectrolyte ER fluids. However, PILs with linear backbone are limited in practical applications due to low glass transition temperature. Crosslinking is an effective method to overcome this problem but the structure-property relationship in crosslinked PIL ER system is not fully understood. Herein, a family of self-crosslinked poly(bis-vinylimidazolium bis(trifluoromethane sulfonylimide))-based PILs with different length of alkyl spacer (P [CnDVIM] [TFSI]) were synthesized. The influence of alkyl spacer length on ER effect was studied and the influence mechanism was analyzed by combination of rheology, dielectric spectroscopy and X-ray scattering measurements. It shows that as alkyl spacer length increases, the ER effect increases but the working temperature range becomes narrow. This is related to the fact that the length variation of alkyl spacer alters the mesh size of crosslinking network of P[CnDVIM] [TFSI], which further influences the transport dynamic of mobile counterions and ion motion-induced interfacial polarization.