Laser Raman spectrum method was used to study hardness index of tap water sample in different stages of waterworks production. Ratio between the bending vibration peak intensity and stretching vibration peak intensity of water samples was measured. Depolarization ratio of laser Raman spectrum at stretching vibration was also calculated. The results show that ratio between the bending vibration intensity and stretching vibration intensity of water samples decreases with the decrease of the samples’ total hardness, and depolarization ratio of laser Raman spectrum at stretching vibration decreases too.