Abstract: The energy of loading stability is analysed for testing machine-sample system with variationalprinciple,and criteria of stable and nonstable loading are set up.It is showed that the parameters of thetesting machine-sample system may be made to suit to the criteria of stable and nonstable loading byadjusting loading methods. These criteria may be used as the theoretical basis for servo-control testingmachine,and also may be used in the stability analyses of country rock-pillar system.