Abstract： Objective To investigate effects of simulated heliox diving on electrocardiogram (ECG) in divers at different high altitudes.Methods In the dual-purpose hyperbaric and hypobaric chamber complex,4 divers conducted a series of simulated dives for a succession of 9 days at various altitudes and under different simulated environmental conditions:the 30 m dive at an altitude of 3000 m,the 30 m dive at an altitude of 4000 m,and the 30 m and 50 m dives at an altitude of 5200 m.Monitoring of ECG was made at different pressure points,and heart rate,P-R interval,QRS vector,Q-T interval,RV5 + SV1 potentials were measured accordingly.Results High altitudes and diving could increase heart rate and shorten P-R and Q-R intervals of the divers.During the experiment,no changes were detected in P wave,ST-T waveform,QRS vector as well as potentials.Conclusions Increased heart rate seemed to be the main effect of the simulated heliox diving at different high altitudes on ECG of the divers.