The present invention provides a demapping device and method. The device includes: an averager receiving a compensation data for determining a degree of the compensation; a demapping function module receiving the compensation data for demapping the compensation data and then outputting a demapped compensation data; a quantizer coupled to the averager and the demapping function module, utilized to quantize the demapped compensation data according to the degree of the compensation and a maximum of quantization; a counter coupled to the quantizer for receiving a quantized data outputted from the quantizer, utilized to count a maximum value and a minimum value within the quantized data and output a counting result, and to simultaneously provide the maximum of quantization for the quantizer; and a compressor coupled to the counter, utilized to compress the counting result to reduce a number of bits.