A measurement system of respiration-related signals includes a plurality of electrodes (11, 12, 13), an ECG processing device (14), and an analysis device (15). The ECG processing device (14) acquires a first analog differential signal and a second analog differential signal through the electrodes and generates a Lead-I ECG signal and a Lead-II ECG signal which is orthogonal to the Lead-I ECG signal after processing the two analog differential signals. The analysis device (15) executes a trigonometric function to calculate angle change values of a cardiac axis (7) based on the acquired Lead-I ECG signal and the Lead-II ECG signal, and further generate a cardiac axis angle change waveform (82) according to the continuous angle change values. Accordingly, the cardiac axis angle change waveform (82) is suitable for the prediction of respiratory conditions of the participant because of the high positive correlation between the frequency of the cardiac axis angle change waveform (82) and the human respiratory frequency.
