The invention, which belongs to the technical field of lithium ion battery cathode materials, discloses a porous carbon in situ composite lithium iron phosphate cathode material and a preparation method thereof. A synthetic process mainly comprises the following steps: dispersing porous carbon; compositing a precursor FePO4.2H2O/porous carbon, and synthesizing the product of the porous carbon in situ coating lithium iron phosphate cathode material. The porous carbon in situ composite lithium iron phosphate cathode material comprises 0.5 to 15 wt% of the porous carbon, 85 to 99.5 wt% of lithium iron phosphate, and 0 to 10 wt% of residual carbon obtained after organic carbon source pyrolysis. Most lithium iron phosphate particles are restricted in the inner of pores of carbon through in situ coating, so obtained composite cathode material particles have small diameters and a good uniformity. Carbon walls of porous carbon allow uniform coating of the lithium iron phosphate particles to be realized and simultaneously particle agglomeration to be largely reduced. The porous carbon in situ composite lithium iron phosphate cathode material has the advantages of good charge-discharge performance, stable circular property, rate capability, novelty, simple synthetic technology and easy large-scale production.