Novel lignocellulosic composite films (LCFs) based on poplar wood flour (PWF) and waste filter paper (WFP) were developed with 1-allyl-3-methylimidazolium chloride ([Amim]Cl) as green solvent. Scanning electron microscopy revealed that the dissolved part acted as the matrix that combined with the insoluble part and reinforced the LCFs. Under the same preparation conditions, there was an increased amount of the insoluble part with an increased amount of PWF. The PWF and WFP mass ratio was a key parameter that influenced the solubility of the raw materials. The differences in the grammage, thickness, and mechanical properties among the groups were related to the dissolution extent of the raw materials. Group A (100 wt% WFP) had the best mechanical properties because of the highest dissolution of cellulose. When the PWF and WFP mass ratio increased, the mechanical properties of the samples decreased and surface bonding became poor. Consequently, the TS of group F (100 wt% PWF) was the lowest. The elongation and elastic modulus showed the same trend as that of the TS.