A handheld device casing with button module includes a shell, a button module and a handheld computer. The button module is disposed in the shell. The button module has a main portion. The main portion has a first penetrating groove, a first triggering structure and an elastic arm. The first triggering structure is disposed to one end of the first penetrating groove. The elastic arm is disposed in the first penetrating groove. A bottom surface of the elastic arm has a second triggering structure. The handheld computer is disposed in a middle of the shell. The handheld computer has a first function button and a second function button. The first triggering structure is corresponding to the first function button. The second triggering structure is corresponding to the second function button.