Difference of stress distribution,maintaining condition and long age between the concrete lining of road tunnel and permitted qualification of synthetic method of ultrasonic rebound are analyzed,and a principle is put forward that the lining strength should be deduced according to the classification of the lining of road tunnel based on the grade of surrounding rock.A revised formula of three parameters for calculating lining strength is derived after the comparison of 7 formulas,which could well consider the stress in lining,the long age and the maintaining condition of lining concrete.