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To explore the spatial variability of soil salinity at root zone in cotton field under plastic drip irrigation, we conducted field experiments in 2013 and 2014 and employed the coefficient of variation, mean relative deviation and standard deviation to investigate the spatial variability of soil salinity for 0~5cm, 5~10cm, 10~15cm, 15~20cm, 20~30cm and 30~40cm layers, and further confirmed representative points that well reflected average soil salinity for each layer. Results showed that for 0~40cm depth, variation degree of soil salinity of only a few points were strong, and most points were medium. Furthermore, in this main root zone, temporal stability of soil salinity first obviously increased and then slightly decreased with increasing of depth. Its stability was the strongest in 30cm depth under surface, with the smallest floating range of mean relative deviation and the smallest mean standard deviation. Based on these findings, some representative points could be selected to estimate average soil salinity for 0~5cm, 5~10cm, 10~15cm, 15~20cm, 20~30cm and 30~40cm layers, and the spatial distribution of the selected points was concentrated. Besides, the relationship between the salinity of representative points and that of the whole area showed a good linear correlation, with high accuracy and determination coefficient from 0.7912 to 0.9171. This research on temporal stability of soil salinity in main root zone can help to guide the field irrigation. And selecting as few representative points as possible can provide a theoretical basis for observation points of soil salinity in this region.