A natural bipolar cloud-to-ground lightning with two return strokes has been recorded with a slitless spectrograph. The subsequent positive stroke occurred retracing the preexisting channel created by the first negative stroke. They both were followed by a long continuing current going with M components. The spectra of the discharge channel for the M components were recorded first with a spectral range from 400 to 1000nm. The spectral analysis suggests that the polarity of the discharge and the direction of the current have little effect on the structure of the spectrum, while the discharge strength, including current magnitude and duration, is the main factor determining the structure of the spectrum. The spectra of the negative and the positive return strokes are characterized by strong NII lines within the visible range, especially the two representative lines of 500.5nm and 568.0nm. The spectra of the M components were mainly from the contribution of OI and NI emissions in the infrared range, and typically, the four strong lines of OI 777.4nm, NI 746.8, 821.6, and 868.0nm were persistently observed in the spectra of the whole lightning process. In addition, the spectral characteristics in different channel sections show that background intensities for both the return strokes and all the M components have an increasing trend with increasing height along the channel. However, the intensities of NII lines for both the return strokes decrease slightly with increasing height along the channel, which should be related to the transmission characteristic of the current in the discharge channel.