To study the freezing and thawing diseases and deformation characteristics of the high-speed railway subgrade in the seasonal freezing area, the K1934+190 ballastless track subgrade section of the Lanxin high-speed railway is used as the research object. With the existing governing differential equations of water transfer and transient temperature field, as well as the stress-strain equation of soil elements, a mathematical calculation model for the coupled field of the ballastless track subgrade is proposed to compare and analyze the dynamic changes of the subgrade from the beginning of October 2019 to the beginning of May 2020 through simulating and monitoring on site. The results indicate that the variation of moisture and temperature of subgrade caused by the coupling effect of moisture field and temperature field on high-speed railway subgrade in seasonal freezing regions, and the resultant redistribution of temperature field and water field are main reasons for frost heaving and thawing settlement of the subgrade. And it can be concluded that the deformation changes regularly in time domain. This conclusion is expected to provide guidance and reference for further studies on frozen damage control of the high-speed railway subgrade in the frozen soil area.