This paper focuses on the impedance measurement method based on spread spectrum time domain reflectometry (SSTDR), aiming to improve the accuracy and applicability of impedance measurement in complex environments. SSTDR achieves high precision impedance measurement in the wide band range by sending a wide band spread spectrum signal and analyzing the impedance change by using the reflected signal characteristics. Through theoretical analysis and simulation verification, this paper reveals the influence law of SSTDR signal parameters (such as PN code length and frequency) on the measurement accuracy and frequency range, which provides a theoretical basis for the selection of signal parameters. Then, a high-speed impedance measuring device is designed based on FPGA technology. The experimental results show that the measurement error of the device is less than 2% in a wide band near the center frequency of SSTDR signal, which verifies the feasibility and superiority of the SSTDR impedance measuring method.