The invention discloses a method for testing a graded broken stone true triaxial apparatus in a mixed boundary loading mode. The graded broken stone true triaxial apparatus comprises a pressure chamber assembly and a loading system. The testing method comprises the steps of placing a pressure chamber base on a vibration table; on the pressure chamber base, forming a cubic chamber through mutual lap joint of four rigid loading plates and four combined loading plates; forming a sample through a vibration method in the cubic chamber; after the sample is formed, placing a top loading plate, top rigid loading ends and a pressure chamber cover plate to integrally form the pressure chamber assembly; transferring the pressure chamber assembly between a loading system base plate and a loading system top plate; after the pressure chamber assembly is in position, adjusting five servo loading systems, so that the five servo loading systems are in contact with the corresponding rigid loading ends; and according to a predetermined test scheme, controlling the five servo loading systems to finish the sample loading process. According to the method, the structure is simple, the size of an applicable sample is large, the three-dimensional load application can be finished, and the load intensity is high.