The measurement of tiny force is an important function of the optical tweezers, for which the trap stiffness plays a key role in the measurement. In this paper, the results of the experimental study on the regulation are reported that the trap stiffness changes with experimental conditions. The results show that the trap stiffness is linearly dependent on the laser power and decreases with the trap height increasing of the bead. In addition, it decreases along with the diameter increase of the trapped bead. The results are qualitatively discussed. The accuracy and range of force measurement are also analyzed.