1. A wearable electrocardiogram measurement device comprising: a heart detection unit adapted to be mounted to a skin of a human body for detecting an activity status of a heart of the human body to obtain electrocardiogram data; and a host portable by a user, with the host including a data storage unit and a processor, with the data storage unit configured to store compressed electrocardiogram data, with the processor electrically connected to the heart detection unit and the data storage unit, and with first, second, and third electrocardiogram compressing programs and a distortion default value being written into the processor, with the first electrocardiogram compressing program executable by the processor to proceed with bandwidth decomposition of sensed data obtained from the heart detection unit, obtaining a plurality of bandwidth data, with the second electrocardiogram compressing program executable by the processor to proceed with quantization of the plurality of bandwidth data respectively by using a plurality of quantization scales to obtain a plurality of quantized results, with an integer part of each of the plurality of quantized results being set as a quantized datum, with a decimal portion of each of the plurality of quantized results being set as a truncated error datum, with each of the plurality of quantization scales multiplied by a corresponding truncated error datum to obtain a product, and with a distortion value obtained by adding the products, wherein when the distortion value is not larger than the distortion default value or fulfills a distortion default value range, the third electrocardiogram compressing program is executed, wherein when the distortion value is larger than the distortion default value or does not fulfill the distortion default value range, a value represented by each of plurality of quantization scales is changed, and the second electrocardiogram compressing program is re-executed, and wherein the third electrocardiogram compressing program is executable by the processor to encode the quantized data to obtain the compressed electrocardiogram data, and the compressed electrocardiogram data are sent to the data storage unit for storage.