A wearable electrocardiogram measurement device and a method for compressing an electrocardiogram are disclosed. A heart detection unit detects a heart activity status of a human body to obtain electrocardiogram data. A processor of a portable host proceeds with bandwidth decomposition and quantization by using quantization scales to obtain quantized results. An integer part of each quantized result is set as a quantized datum. A decimal portion of each quantized result is set as a truncated error datum. Each quantization scale is multiplied by a corresponding truncated error datum to obtain a product. If a distortion value obtained by adding the products fulfills a distortion default value range, the quantized data are encoded to obtain compressed electrocardiogram data for storage in a data storage unit of the host. If the distortion value does not fulfill the distortion default value range, an adjustment step is repeated.