Abstract: AlMg alloy films with a thickness less than 5 m could be used as raw materials of precision cutting for ribbon array load in Z-pinch physics tests. In this paper, the thermal evaporation was used to prepare such ultrathin AlMg alloy films. By controlling the deposition rate of the thermal evaporation craft, AlMg alloy films with a thickness less than 5 m were coated on super-smooth NaCl substrate and after subsequent process of NaCl substrate solution, unbraced AlMg alloy films were achieved. Thickness uniformity, surface roughness, diffraction peak site, crystal grain size and elements component with different distances to surface were measured. It was found that in such a thermal evaporation way, the thickness uniformity of AlMg alloy films was better than 8%, the surface roughness (Ra) of both sides of samples less than 180 nm, the crystal grain size less than 20 nm. It was showed that diffraction peak site of samples with different thickness hardly displaced in the common meaning of few internal stress. It was also showed that the content of Mg element under different depths was stable, the impurity content of film surface was higher than that in the center place, and an anticipative component could be achieved under 6 nm depth stably. Prepared by thermal evaporation, unbraced AlMg alloy films with the characteristics of controllable and uniform thickness, stable component and few internal stress were applicable for ribbon array load of Z-pinch.