Having set up the mechanical structure motive reliability functional equation with random load. The reliability can be worked out using the second-matrix and perturbation methods. At the same time, the variation regularity of the mechanical structure strength, load, reliability and the failure rate have been studied. It indicates that when the strength decreases, the reliability of mechanical structure inclines slowly with time increasing, and the failure rate increases at the same time, and it has the feature of the later period of a "bath tub "curve. Using a specific example the motive curve of reliability and the failure rate with time changing can be obtained. They have certain guiding significance for determining test time and reliable life of components.