Cultivation after converting saline and alkaline wasteland to agricultural land is usually accompanied by accelerated soil salinization. There is general agreement that freezing-thawing process plays a crucial role in soil salinization in mostly cold-arid agricultural areas, which leads to the impoverishment of soil productivity. To investigate the migration characteristics of soil salinity in saline-sodic cotton field with different reclamation time in non-irrigation season, we examined the soil bulk, total porosity, and spatial and temporal migration of soil salinity in winter in an oasis agroecosystem of northern Xinjiang, China. Experimental plots include a saline-alkali wasteland as control and cotton fields under four age categories (cultivation start years were 1998, 2006, 2008, and 2012, respectively). The results showed that converting saline-alkali wasteland to cotton land significantly decreased soil bulk density but increased total soil porosity, especially at 0–40 cm. Conversion significantly decreased soil salinity at 0–200 cm soil depths, with a greater decrease in cotton land after relatively longer time of cultivation. After freezing-thawing process, soil bulk density decreased 4.87% on average, and total porosity increased 5.98%. Soil salt dynamics showed that there were two stages in salt accumulation. Firstly, soil salt storage increased at 0–100 cm depth but decreased at 100–200 cm depth during freezing period; Secondly, rapid increase of salt storage appeared at late thawing period. In addition, salt accumulation positively correlated to initial salt content and bulk density, but negatively correlated to total porosity. At springtime, cotton land with short-term cultivation was desalination comparing with pre-freezing, but salt accumulation occurred after long-term cultivation. These results highlighted that long-term cultivation with mulched drip irrigation decreased soil salinity in the 0–200 cm soil profiles, but freezing-thawing process induced soil salinization which poses a potential risk to agricultural sustainability.