To study the flexural performance of 5.1-meter-long cement-based plant fiber fireproof and thermal insulation prefabricated composite slabs, this paper conducted four-point bending tests on four full-length composite slabs, analyzed their failure characteristics, crack characteristics, strain laws, ultimate bearing capacity and ductile coefficient, and discussed the influence of different protective layer thicknesses under positive and negative bending moment loading. The results show that the specimens with positive and negative bending moments present different failure characteristics. The prominent manifestation is that the prefabricated bottom plate of the specimens with positive bending moments shows no obvious phenomenon before failure and presents brittle failure characteristics. At this time, the load will have a certain amplitude of "steep drop", and eventually the crushing phenomenon occurs in the compression zones of both. This composite long plate is lightweight, has good bearing capacity and good ductility. Among them, the positive bending moment specimen has a higher ultimate bearing capacity than the negative bending moment specimen by approximately 50% due to the high tensile strength of the prefabricated bottom plate in the tensile zone and about 23% higher than the cast-in-place slab with the same cross-section. The negative bending moment specimen has a lower bearing capacity than the cast-in-place slab due to the lower compressive strength of the plant fiber cement-based material in the compression zone. The prefabricated base plate placed in the tensile zone can significantly inhibit crack propagation. The crack spacing of the specimens with positive bending moment is 20% to 30% smaller than that of the specimens with negative bending moment. Finally, based on the assumption of the flat section and by using the existing specification formulas, the loading mode coefficient was introduced and a correction formula for the cracking bending moment was proposed. The error between the theoretical calculation value and the test value is less than 5%. The research shows that this correction formula can provide a reliable basis for the calculation of the cracking bending moment of this new type of composite slab.