Herein, novel, loose, and porous graphite phase carbon nitride/porous carbon (g-C3N4@PC) composites were prepared by decorating cellulose nanocrystals (CNCs). The characterization results demonstrate that the as-prepared composites presented high specific surface areas, porous structures, and abundant chemical groups, with the modification of CNCs. In view of the unique advantages, g-C3N4@PC was used as the coating material for the solid-phase microextraction (SPME) of organochlorine pesticides (OCPs) in water and juice samples. The g-C3N4@PC-coated fibers showed better extraction efficiencies than commercial fibers (100/7 μm PDMS and PA) toward the OCPs, with the enrichment factors of the g-C3N4@PC-coated fibers 5–30 times higher than the latter. Using a gas chromatography–mass spectrometry (GC-MS) instrument, the g-C3N4@PC-coated fibers exhibited a gratifying analytical performance for determining low concentrations of OCPs, with a wide linear range (0.1–1600 ng L−1for water; 0.1–1000 ng L−1for juice), low limits of detection (0.0141–0.0942 ng L−1for water; 0.0245–0.0777 ng L−1for juice), and good reproducibility and repeatability in optimal conditions. The established method showed good sensitivity and recovery in the determination of OCPs in the water and fruit juice samples, which displayed broad prospects for analyzing organic pollutants from environmental samples.