Abstract: Due to large hydraulic loading force and the difficulty of keep normal stress constant,there are many defects in conventional direct shear apparatus,such as the inadequacy of small load rock joint and low accuracy of shear strength results.Based on consistency of principle of instrument and reliability of test result,a stress effect direct shear apparatus of small load rock joint has been developed by the principle of mechanical lever.The newly developed direct shear apparatus were mainly composed of platform,loading system,distortion measuring system,real-time image displaying system and so on.It is not only simple in structure and convenient in operation,but also low in cost and high precision.The small load direct shear apparatus was compared and analyzed in the principle of instrument,test period and result of direct shear test.In order to verify the reliability of the direct shear apparatus,the results of direct shear test were compared with the result of PFC.The main results of study are as follows:The small load direct shear apparatus has higher accuracy for small load rock joint.The principle of mechanical lever was applied to the direct shear apparatus to keep normal stress constant,which can ensure the reliability and stability of rock joint shear strength.Compared with the conventional direct shear apparatus,the small load direct shear apparatus can shorten the test period of about 85%.