The high value-added utilization of fluid catalytic cracking (FCC) slurry has always been full of challenges and has important practical significance. Ultralight N-rich porous graphene nanosheets (N-PGNs) have been synthesized via a chemical vapor deposition method using FCC slurry as a carbon source and g-C3N4 as a substrate. The N-PGNs integrate interesting features including high N doping and large surface area with ultrahigh pore volume, which leads to intriguing performance in electromagnetic wave absorption (EMWA) along with broad absorption bandwidth. When the ratio of FCC slurry and g-C3N4 continues to optimize, the maximum reflection loss of the N-PGNs can reach -53 dB with a low filler loading of only 3 wt %, and the effective absorption bandwidth exceeding -10 dB is 6.8 GHz with a thickness of 3 mm. The excellent property of the N-PGNs and their extraordinary EMWA performance develop a new pathway for the high value-added utilization of FCC slurry.