A staple gun with a safety device and its safety device are disclosed. The staple gun with a safety device comprises a safety device and a staple-floating unit that are installed in a housing of the staple gun. The safety device includes a safety push sheet and a switch. In a middle portion of the safety push sheet is defined an engaging slot along a staple-discharging direction, and the safety push sheet is further provided with a triggering sheet extending from an upper end thereof. The staple-floating unit is movably connected to the safety device and includes a guide sheet, a control knob, and a positioning pin. The positioning pin can be used to drive the safety push sheet and the guide sheet to move downwards to make the safety push sheet and the guide sheet to protrude out of the staple exit synchronously for facilitating exerting the staple-floating function.