The invention discloses a thulium-based metallic glass and its preparation method and application. The thulium-based metallic glass is mainly composed of thulium, and its composition is shown in formula (I), wherein, RE means selected from Ce, Pr, Nd, One or more rare earth elements in Sm, Gd, Tb, Dy, Ho and Er; T represents Co or Ni; a, b, c, d represent the atomic percentage of each element, 30≤a≤60, 5≤ b≤30, 20≤c≤25, 15≤d≤25, 50≤a+b≤60 and a+b+c+d=100. According to the thulium-based metallic glass of the present invention, the change of RE and T can modulate the amorphous forming ability on the one hand, and on the other hand can modulate the temperature range in which the magnetic transition of the thulium-based metallic glass occurs, broaden the magnetic transition region, and make it work in a wider range. It has a large magnetocaloric effect in the temperature range. TmaREbAlcTd (I).