The invention discloses an impedance analyzing method for sub-synchronous oscillation of a thermal power unit. The method comprises the following steps: partitioning a system to be researched into a power supply and a power grid, building state space models in sequence for the power supply and the power grid, and calculating admittance function matrixes of the power supply and the power grid; then, drawing Bode diagrams of the admittance function matrixes of the power supply and the power grid on the same graph; and lastly, picking up apparent resonance points on the bode diagrams, and calculating system series connection resonance frequency and an excited axis modal according to the apparent resonance points. Based on the basic principle of the impedance analyzing method, the power supply of the thermal power unit is modeled uniformly in a state space form, and the dynamic features of a mechanical control system of the unit are included in the state space of the power supply and modeled finally in the form of electric quantity. Compared with a conventional method, the method has the advantages that sufficient calculation accuracy can be obtained, and the dynamic features of continuous frequencies can be analyzed, so that beneficial conditions are created for the stable research of an electric-electronic power system.