Flat dilatometer with external friction angle testing function and testing method

The invention discloses a flat dilatometer with an external friction angle testing function and a testing method, and belongs to the technical field of soil mechanical property testing. The flat dilatometer comprises a probe part, wherein a steel diaphragm is arranged on the side surface of the probe part; the bottom end of the movable rod part is connected with the probe part, a limiting groove is formed in an inner cavity of the movable rod part, and the top end of the movable rod part can axially and movably sleeve the bottom end of the fixed rod part; the lifting part is arranged in the limiting groove in a lifting manner and is connected with the fixed rod part in a relative movement manner; and the force measuring part is arranged between the lifting part and the movable rod part and is used for acquiring static friction force acting on the steel diaphragm. According to the invention, expansion point values of a plurality of points on the steel diaphragm can be measured at the same time, the testing fun