Because the loose circle is an important basis and parameter for designing the tunnel support and estimating whole stability, how to determine its area accurately and fast has become all issues of concern. By taking the tunnels inner highway from Zhangjiakou to Shijiazhuang as study background, the loose circle of tunnels with different overburden depth and rock mass grade was studied by combination of ultrasonic detection and theory analysis. Based on the criterion of Drucker-Prager, the calculation method of theoretic analysis for the loose circle of surrounding rock mass is founded, whose calculated results is lower than those from ultrasonic detection of single hole but has similar variation rules each other. For considering the strain softening after peak strength, the analysis method based on damage theory more approach to results from the field test than the ones of elastoplasticity. The estimation of loose circle of surrounding rock mass with damage theory, especially for low graded rock mass, is feasible. Besides, the study results have some consult effects on the similar engineering.