Based on the composite element method, we propose a composite element model associated with a three-dimensional heat conduction-convection equation and a “filled model” for the determination of the transient thermal field in fractured rock mass. Although the existence of fractures is not considered during mesh generation, it is explicitly treated in the mapping composite element by means of the composite element pre-processing work program. There is no restriction on the computational mesh generation. A self-adjoint process is made to adjust the heat conduction-convection equation. The composite element algorithm is deduced by applying the variational principle for solving transient thermal field in fractured rock mass. The proposed method is applied to calculate the respective temperature of rock sub-elements and fracture segments, in which it also takes into account the thermal exchange characteristics of the fluid in fracture and the adjacent rock blocks. The obtained numerical results are consistent well with the experimental data. The reliability and validity of the proposed numerical method are verified with the measured results, which show that the thermal conduction and convection and obviously occur between the fluid in fracture and the adjacent rock blocks.