The invention discloses a continuous density gradient low-density porous carbon bonding composite material.The composite material is prepared from, by mass, 12 parts of chopped viscose-based carbon fiber, 7-36 parts of fiber binder and 1.5-5 parts of fiber dispersing agent.A preparing method of the composite material comprises the steps of 1, preparing fiber size and conducting vacuum filtration formation on the fiber size; 2, heating and drying wet green obtained in the step 1 till weight does not change any more, and curing a mixture obtained after drying with the gradient curing technology under the vacuum condition; 3, conducting furnace cooling on the mixture obtained in the step 2 to indoor temperature, and then carbonizing the mixture obtained in the step 2 with the gradient carbonization treatment technology.The composite material has a high temperature resisting property and a heat insulating property, has a continuous density gradient feature in the thickness direction, and has high mechanical strength, anti-scouring performance and thermal protection/insulation efficiency.