Abstract： Objective To record,analyze and evaluate the ECG of oceanauts operating the "Jiaolong"deep-sea manned submersible.Methods During underwater operation,the ECG signals of the 5 oceanauts were collected by using MP30 electrocardiograph (Biopac System Inc.) for ECG detection.The 6 indexes of heart rate (HR),P-R interval,QRS duration,Q-T interval and the voltage of R and T waves were recorded and analyzed.Results The HR control value on the surface before submergence was(61.0 ± 2.9) times/min.At hour 1.5,2.5 and 3.5,the heart rates of the oceanauts operating the "Jiaolong" deep-sea manned submersible respectively increased to(89.1 ± 12.1),(86.2 ± 10.2) and (83.3 ± 10.3) times/min,and statistical significance could be noted,as compared with that before submergence (P ＜ 0.05).At hour 1.5,2.5 and 3.5 during underwater operation,the P-R intervals respectively shortened to (168.1 ± 12.1),(167.2 ± 12.2) and (181.3 ± 11.3) ms,as compared with the control value on the surface(196.1 ± 11.9)ms.Statistical significance could be noted,when comparisons were made between them(P ＜ 0.05).At hour 1.5,2.5 and 3.5 during underwater operation,the QT intervals respectively shortened to(400.2 ± 29.2),(386.1 ± 19.1)and(393.0 ± 13.0)ms as compared with the control value on the surface (444.4 ± 13.4) ms,also with statistical significance (P ＜ 0.05).There was no statistical significance in QRS duration,R and T wave amplitudes at hour 3.5 during underwater operation (P ＞0.05).Conclusions During underwater operation,the changes in ECG of the oceanauts mainly displayed in the increased heart rate,and shortened PR and Q-T intervals.However,there were no significant changes in QRS duration,R and T waves,indicating that the ECG of the oceanauts was on the whole still within normal level,though cardiac back-up of the oceanauts during submerged operation was decreased to some extent.