As a typical harmonic source, the single-phase rectification loads will produce harmonic pollution into power grid, which will further cause the harmonic power loss. Based on the operation principle of the single-phase rectification loads, their harmonic active and reactive powers and the flow direction are investigated. In consideration of the coupled relationship between the terminal current and voltage harmonics of rectifiers, their power demand models are established to calculate the harmonic power accurately. The correctness of the model is verified by the simulation. On this basis, the admittance matrices of harmonic active and reactive power are defined. And the contribution degree that the voltage harmonics of single-phase rectification loads act on their harmonic powers is studied through the analysis of the physical significance and the elements of admittance matrices.