This paper presented a new method based on down-sampling for QRS complexes detection for the reason that ECG's frequency components mostly centralize 0.05-35 Hz. Firstly, the original ECG signals were down-sampled. Secondly, self adapting and self study theory were used to select the amplitude threshold and slope threshold for R-wave detection. At the same time, the refractory idiosyncrasy of heart was taken into account to reduce the misdetection. Thirdly, back-trace technique was used to reduce undetected error. Finally, the corresponding R-wave position was up-dated in the original ECG signals. The performance of the method was tested by MIT/BIH ECG database and the clinical data were collected from the affiliated hospital of Chongqing Medical University. The detection rate of 98% was achieved. Experimental results indicated that the method was simple, effective, accurate and unaffected on sample rate.