We study effect of addition of graphene on mechanical properties (elastic modulus and Poisson's ratio) of hydroxyapatite composites. Firstly, generating algorithm of random distribution geometric model of graphene/hydroxyapatite composites was created and model's automatic generation program was developed. Then, finite element model of graphene/hydroxyapatite composite was established. Finite element analysis of uniaxial tensile tests of composite materials was carried out. Finally, mechanical properties of different mass fraction of graphene of composite were calculated. Validity of the algorithm is verified by experimental data. It shows that elastic modulus of composite increased 12% to 50% with addition of 0.25% to 1.25% (wt.) graphene. Mechanical properties of hydroxyapatite can be effectively improved by addition of graphene.