The purpose of this study was to use inertial sensors to develop a method for analyzing the motion skills in swim. Through the proposed analysis method, the correlation between sensor reading and the motions of swim was examined. A total of 11 college swimmers were participated in the experiment. Water-proof portable devices and self-developed programs were used to extract data from the triaxial accelerometers and gyroscope installed on the hands and back of the swimmers, respectively. The data obtained from the sensors were presented using waveform graphs. The major research findings are as follows: (1) the characteristic analysis of accelerometer waveform reveals the basic parameters data, such as stroke frequency, stroke length, swim speed, and stroke efficiency. (2) the gyroscope waveform can be used to calculate and analyze the trunk rotation angle of the swimmer. (3) the accelerometer waveform can be employed to analyze swim postures, and thereby facilitate correcting the swim motion skills. (4) the variability of the correlation coefficients among basic accelerometer parameters data can be adopted to evaluate the motion stability of the swimmers. We concluded that applying sensor reading techniques to scientific research on swim can rapidly and effectively obtain motion parameters data, instantaneously review the motion efficiency of swimmers, and assist swimmers in correcting their swimming skills. The inertial sensor can be used as a novel tool in swim instruction and training.