A method for detecting a preload residual rate involves: a. installing a temperature sensor on one of two preloaded elements; b. making the two preloaded elements to move with respect to each other, and recording a time-related temperature variation sensed by the temperature sensor, so as to obtain an initial temperature-rising curve; c. making the two preloaded elements to move with respect to each other, and recording a time-related temperature variation sensed by the temperature sensor, so as to obtain a detected temperature-rising curve; and d. comparing the initial and detected temperature-rising curves, so as to obtain the preload residual rate between the two preloaded elements of the step c to the step b. The method detects a preload residual rate applied to an object when the object is operating while being advantageous in terms of cost, service life, response and accuracy.