Abstract: The quality of rock mass is a comprehensive reflection of the complex engineering geological characteristics of rock mass, and the basis for stability identification and support design on rock engineering.Most existing rock mass classification methods are based on shallow rock engineering and not suitable for deep rock masses.Considering the characteristics of high ground stress, high temperature and high osmotic pressure in deep underground engineering, the traditional Q-system is selected from the existing rock mass classification methods and modified for the quality evaluation of surrounding rock mass in deep engineering, also, it is applied in an underground mine.The results show that the class of rock mass using improved Q-system is generally one level lower than that by the conventional Q-system.So the conventional Q-system has a hidden safety hazard in evaluating the quality of deep rock mass and must be revised.