The capability of a high-temperature superconducting sigma-delta modulator was studied by means of circuit simulation and FFT analysis. Parameters for the circuit simulation were extracted from experimental measurements. The present circuit simulation includes thermal-noise effect. Successive FFT analyses were made to evaluate the dynamic range of the sigma-delta modulator. As a result, the dynamic range was evaluated as 60.1 dB at temperature of 20 K and 56.9 dB at temperature of 77 K.