Research conclusions ： (1) The main reasons for the four - comer seams of CRTS - I ballastless track slab are the buckling deformation caused by the temperature difference between the board and the bottom and the Hanging Empty phenomenon between lower and higher tracks among adjacent slabs after track — laying. (2) The night or the season of least temperature difference between day and night would be better to do mortar pouring, and the best time is between 21:00 and 6 :30. Also delicate adjustment, effective clamping force and full mortar pouring are necessary. (3 ) It is also necessary to improve the adjustment accuracy of the slabs, especially to maximize to eliminate the height difference of adjacent tracks. (4) This study is very useful to reduce the seams of CRTSI,II,IH ballastless track slabs four comers.