To enhance the field emission performance and understand the emission essence of carbon materials, carbon films with different morphology and phase composition were fabricated by regulating the reactive gas flow ratio in a hot filament chemical vapor deposition system with the assistance of electric field and magnetic field. The characterization of carbon film quality and measurement of their field emission performance indicated that carbon nanostructures, including nanoclusters, nanopillars and nanotips, which were determined by the methane concentration in the process, were formed under the assistance of electric field. Further magnetic field assistance could lead to the formation of finer nanostructures with higher crystallinity. Field emission performance was highly dependent on the morphology and phase composition of the carbon films. Among all the samples, carbon nanotips, deposited in high methane concentration with the co-assistance of magnetic and electric field, possessed the best field emission performance, with the lowest turn-on field of 6.5 V·μm−1 (J = 10 μA·cm−2). Besides, a comparative study on the fitting degree of several conduction mechanisms revealed that the essence of field emission was the combination of several emission mechanisms, which was also dependent on the morphology and phase composition of these carbon films. Display omitted •An alternative growth method for yielding carbon nanomaterials was proposed.•The process-dependent formation mechanism was studied.•Enhanced field emission performance was realized by adding external fields.•Essence of field emission was proved to be a combination of several mechanisms.