Abstract: An unconstrained cardiac output detection system based on thoracic electrical impedance method and embedded technology is developed in this study. PWM function of PIC embedded MCU, PIC18F4520, and filter circuit are used to generate a constant current sinusoidal signal with a frequency of 50 kHz and a current RMS of 2 mA_rms, which is applied as an exciting signal to the exciting electrodes located on the positions behind ears and on right lower abdomen. The thoracic electrical impedance is measured by the detecting electrodes pasted along the medial line on the front of the thorax, and is used to calculate cardiac output. A comparative experiment with the thoracic impedance method and the dye dilution method is carried out and the results are investigated. The experimental results show that the two methods have good consistency.