Abstract: To address the issues of the inherent flammability and the short alarm duration of graphene oxide (GO) in fire warning applications, a flame-retardant fire warning composite film (MCNC/GO) was prepared via a self-assembly process based on GO modified with phosphorus-nitrogen hybrid cellulose nanocrystals (MCNC). MCNC was obtained by the hybrid modification of cellulose nanocrystals (CNCs) with phytic acid and aniline, and the effects of different mass fractions of MCNC on the micromorphology, thermal stability, flame retardancy, and fire warning behavior of the GO films were investigated. The results show that MCNC is uniformly dispersed between the GO layers and constructs an effective intercalated structure, and the composite film exhibits good flexibility. Compared with the pure GO film, the peak heat release rate (PHRR) of the composite film containing 44.44wt% MCNC decreases by 82.76%, and the total heat release (THR) decreases significantly, demonstrating superior flame retardancy. In the fire warning tests, the MCNC/GO composite film triggers an alarm at approximately 4 s after flame contact, and the alarm duration reaches 807 s. The mechanism analysis indicates that the introduction of MCNC not only enhances the stability of char formation of the composite through catalytic charring but also promotes the rapid reduction of GO in the early stage of heating, thereby achieving an efficient and persistent fire warning function.