Mathematic models of the fiber-optic distributed disturbance sensor (FDDS) based on the phase-senstive optical time-domain reflectometry (OTDR) are established and the influences of the frequency drift of the laser on the sensor are simulated and experimented based on multiple-wave theory. The simulation results demonstrate that the frequency drift of the laser is a critical factor that results in the reduction of the positional accuracy of sensor and the signal-to-noise ratio (SNR). The SNR is less than 2 dB by moving averaging and subtraction of the traces with disturbances from traces without disturbances and by moving averaging and moving differential method when the frequency drifts of the laser are larger than 25 MHz/min and 30 MHz/min, respectively. It means that the two location methods fail to locate the disturbance. Experiments are made with lasers with frequency drifts of 3 MHz/min and 19 MHz/min, respectively. The experimental results show that the location errors with the two location techniques are both 100 m when the laser with a frequency drift of 3 MHz/min is utilized. In contrast, it fails to locate the disturbance when the frequency drift of the laser is 190 MHz/min. The investigation results are helpful to select the laser source and improve the location precision of the sensor.