The present invention relates to a sheet stack thickness estimating device including a driving device, a first connecting rod, a second connecting rod, a pressing part and a sensor. The driving device provides a motive force to the sheet stack thickness estimating device so as to drive the first connecting rod moved horizontally for rotating the second connecting rod, so that the pressing part presses the sheets on the paper tray. By detecting if the signal generated by the sensor is blocked by the first connecting rod, the thickness estimating device can judge if the thickness of the sheets exceeds a predetermined value.