1. A relaxation state evaluation system comprising a sensor module measuring electrocardiograph (ECG) data of a user; and an evaluation module electrically connected with the sensor module, analyzing the ECG data to generate a first parameter, a second parameter, a third parameter and a fourth parameter, and working out an evaluation index, wherein the first parameter is a short-scale entropy slope before a cardiovascular disease treatment (CVDT) of the user; the second parameter is a difference of a post-CVDT mean RR interval and a pre-CVDT mean RR interval; the third parameter is a logarithm of a variance of a pre-CVDT high frequency normal to normal (NN) intervals; the fourth parameter is a logarithm of a ratio of a variance of a pre-CVDT low frequency NN intervals to the variance of the pre-CVDT high frequency NN intervals; the evaluation index is a function of the first parameter, the second parameter, the third parameter and the fourth parameter; the user is determined to in a relaxation state if the evaluation index is over a threshold.