In force calibration, when triangular-wave input is exerted on the piezoelectric-driven substage, there is a damped oscillation superposed on the square-wave movement of the trapped bead in our experiments. This will bring about instability for well trapping bead and become the noise source of the bead signal. Consequently, the superposed vibration will influence the precision and the range of force that can be calibrated. In order to analyze derivation of the oscillation and solve it, a model of equivalent oscillator is put forward. Based on this model, we calculate the initial amplitude of oscillation and frequency spectra of the movement of trapped bead, which is well in agreement with the experimental one. We apply sinusoidal-wave input substituted for triangular one to the substage, as a result, oscillation disturbance is avoided effectively, so the trapping stability of optical trap increases and the range of calibrated force has been extended greatly.