A device for detecting the thickness of a sheet medium, comprising: a conveying roll, configured to convey a sheet medium; a detecting roll, arranged opposite to the conveying roll, configured to clamp and convey the sheet medium together with the conveying roll in the process of convey and is elastically displaced relative to the conveying roll; a sensor, configured to obtain a distance variation value between the detecting roll and the sensor; a thickness calculating unit, configured to calculate a sheet medium thickness value according to a distance detection value between the detecting roll and the sensor obtained by the sensor when the sheet medium passes by and a standard zero value between the detecting roll and the sensor obtained by the sensor before the sheet medium enters; wherein the device for detecting the thickness of a sheet medium further comprises: a zero value correcting unit, configured to correct a dynamic detection zero value between the detecting roll and the sensor obtained by the sensor before the sheet medium enters according to a zero value correcting formula to obtain the standard zero value; a data storing unit, configured to store a preset distance detection zero value and operating data used by the zero value correcting unit; characterized in that the zero value correcting formula is as follow: b n = 1 − t × b n − 1 + t × a n t ∈ 0,1 where, a n is the dynamic detection zero value used in detecting the thickness of the n-th sheet medium; b n represents a standard zero value used in detecting the thickness of the n-th sheet medium; b n-1 is a standard zero value used in detecting the thickness of (n-1)-th sheet medium; n is a natural number greater than 0, and when n=1, that is, the detection is performed for the first time, b 0 = A 0, with A 0 being a preset distance detection zero value. a conveying roll, configured to convey a sheet medium; a detecting roll, arranged opposite to the conveying roll, configured to clamp and convey the sheet medium together with the conveying roll in the process of convey and is elastically displaced relative to the conveying roll; a sensor, configured to obtain a distance variation value between the detecting roll and the sensor; a thickness calculating unit, configured to calculate a sheet medium thickness value according to a distance detection value between the detecting roll and the sensor obtained by the sensor when the sheet medium passes by and a standard zero value between the detecting roll and the sensor obtained by the sensor before the sheet medium enters; wherein the device for detecting the thickness of a sheet medium further comprises: a zero value correcting unit, configured to correct a dynamic detection zero value between the detecting roll and the sensor obtained by the sensor before the sheet medium enters according to a zero value correcting formula to obtain the standard zero value; a data storing unit, configured to store a preset distance detection zero value and operating data used by the zero value correcting unit; characterized in that the zero value correcting formula is as follow: b n = 1 − t × b n − 1 + t × a n t ∈ 0,1 where, a n is the dynamic detection zero value used in detecting the thickness of the n-th sheet medium; b n represents a standard zero value used in detecting the thickness of the n-th sheet medium; b n-1 is a standard zero value used in detecting the thickness of (n-1)-th sheet medium; n is a natural number greater than 0, and when n=1, that is, the detection is performed for the first time, b 0 = A 0, with A 0 being a preset distance detection zero value. a zero value correcting unit, configured to correct a dynamic detection zero value between the detecting roll and the sensor obtained by the sensor before the sheet medium enters according to a zero value correcting formula to obtain the standard zero value; a data storing unit, configured to store a preset distance detection zero value and operating data used by the zero value correcting unit; characterized in that the zero value correcting formula is as follow: b n = 1 − t × b n − 1 + t × a n t ∈ 0,1 where, a n is the dynamic detection zero value used in detecting the thickness of the n-th sheet medium; b n represents a standard zero value used in detecting the thickness of the n-th sheet medium; b n-1 is a standard zero value used in detecting the thickness of (n-1)-th sheet medium; n is a natural number greater than 0, and when n=1, that is, the detection is performed for the first time, b 0 = A 0, with A 0 being a preset distance detection zero value. where, a n is the dynamic detection zero value used in detecting the thickness of the n-th sheet medium; b n represents a standard zero value used in detecting the thickness of the n-th sheet medium; b n-1 is a standard zero value used in detecting the thickness of (n-1)-th sheet medium; n is a natural number greater than 0, and when n=1, that is, the detection is performed for the first time, b 0 = A 0, with A 0 being a preset distance detection zero value.