Nanofibrillated cellulose (NFC) can enhance the flexibility and mechanical performance of graphene composite, but there are few researches focusing on dispersibility of the composite via different dispersion conditions, which mainly determine their key properties. This presented work concentrated on the influence of ultrasonic power and time on the property of NFC suspension, graphene suspension, and the composite membrane. With the increase in the ultrasonic conditions, particle size of NFC suspension decreased and the static stability of graphene suspension was improved. NFC-graphene suspension exhibited excellent static stability even adopting the low ultrasonic conditions due to the electrostatic repulsive and adhesive effect among NFCs. After enhancing shearing force induced from ultrasonic waves and cavitation, graphene sheets could be effectively detached and dispersed, and then, the planar uniformity and structural integrity of NFC-graphene membrane tended to be better, which was characterized and confirmed by morphology, chemical, and thermal and phase structure analysis. Conductivity uniformity of the seven points on the membrane exhibited an increasing trend with the increase in the ultrasonic power and time, as well as the mechanical performance, while the heating temperature uniformity had no distinct change due to the excellent thermal conductivity of the graphene. The higher ultrasonic condition was conducive to the stability of electric heating performance. Consequently, the ultrasonic treatment with different conditions had impacted the incorporation of graphene into the NFC matrix. This study's results would be a feasible reference for the improvement of the composite used in various areas.