A device for detecting a thickness of a sheet medium is provided, the device is used to improve the sensitivity of the entire sheet medium thickness detection device while eliminating the zero voltage impact by disposing, before an A/D converter, an appropriately amplified subtraction amplification unit used for eliminating the zero voltage impact of thickness voltage. The sheet medium thickness detection device includes a thickness sensor, a position voltage conversion unit, an A/D converter and a control processor which are sequentially connected, and further includes a medium position detection unit, a timer unit, a D/A converter and a subtraction amplification unit. The medium position detecting unit is connected to the control processor and is used to detect whether a detected medium passes through, and the timer unit is connected to the control processor.