The present disclosure provides a true triaxial tester with adjustable loading stiffness and a test method, relating to the technical field of rock mechanics. The tester includes a first principal stress variable stiffness loading apparatus, a second principal stress variable stiffness loading apparatus, a third principal stress loading apparatus, a monitoring system, and a control system. The monitoring system includes a stiffness sensor, a pressure sensor, and a displacement sensor. Loading stiffness in a first principal stress direction and a second principal stress direction is adjustable. The tester has the following advantages: it has fixed loading stiffness, can complete stiffness loading of one side being free and five sides being variable, and is easy to operate.