A transparent display device (100, 200) comprising: a transparent display unit having a characteristic of transparency and being adapted for displaying a transparent image frame according to a display data signal; a sensing module configured on the transparent display unit, the sensing module being adapted for sensing one of conditions of an environmental background of the transparent display device (100, 200) and a user's input and accordingly outputting a sensing signal, the transparent display device (100, 200) being characterized in that the sensing module comprises: a photographing unit adapted for capturing an ambient image of the environmental background as the sensing signal; and a processing unit coupled to the transparent display unit and the sensing module, the processing unit being adapted for controlling the transparent display unit to adjust transparency of the transparent image frame according to the sensing signal, wherein the processing unit is adapted for calculating the light intensity and a clutter degree of the ambient image according to the sensing signal, for determining a transparency adjustment parameter corresponding to the light intensity or the clutter degree, and for outputting the transparency adjustment parameter to the transparent display unit, so as to control the transparent display unit to adjust the transparency of the transparent image frame, wherein the clutter degree of the ambient image is determined according to an image variation frequency of a predetermined number of display pixels in the ambient image, wherein the image variation frequency is determined according to, for said predetermined number of display pixels, the number of times the differences in gray scale values between pixels are above a specific level; and wherein the processing unit is adapted to reduce the transparency of the image frame if the image variation frequency is above a threshold. a transparent display unit having a characteristic of transparency and being adapted for displaying a transparent image frame according to a display data signal; a sensing module configured on the transparent display unit, the sensing module being adapted for sensing one of conditions of an environmental background of the transparent display device (100, 200) and a user's input and accordingly outputting a sensing signal, the transparent display device (100, 200) being characterized in that the sensing module comprises: a photographing unit adapted for capturing an ambient image of the environmental background as the sensing signal; and a photographing unit adapted for capturing an ambient image of the environmental background as the sensing signal; and a processing unit coupled to the transparent display unit and the sensing module, the processing unit being adapted for controlling the transparent display unit to adjust transparency of the transparent image frame according to the sensing signal, wherein the processing unit is adapted for calculating the light intensity and a clutter degree of the ambient image according to the sensing signal, for determining a transparency adjustment parameter corresponding to the light intensity or the clutter degree, and for outputting the transparency adjustment parameter to the transparent display unit, so as to control the transparent display unit to adjust the transparency of the transparent image frame, wherein the clutter degree of the ambient image is determined according to an image variation frequency of a predetermined number of display pixels in the ambient image, wherein the image variation frequency is determined according to, for said predetermined number of display pixels, the number of times the differences in gray scale values between pixels are above a specific level; and wherein the processing unit is adapted to reduce the transparency of the image frame if the image variation frequency is above a threshold.