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In order to study the characteristics of low frequency pressure pulsation of tubular turbine, the unsteady internal flow field in a tubular turbine of a power station was numerically simulated. The pressure pulsation characteristics of the turbine under different working conditions were also analyzed. The UG software was used to build the model which mainly consisted of inlet conduit, bulb body, guide vane, runner and draft tube. The results showed that the pressure pulsation in the turbine was mainly affected by the rotation frequency of blade (5.26Hz) and the low-frequency pressure-pulse (0.20Hz) under the rated condition and low flow rate condition. The amplitude of the low frequency pulsation at 0.20Hz was increased gradually from the turbine inlet to the outlet. And the amplitude under the low flow rate condition was higher than that of the rated condition. There was a helical vortex rope in the draft tube which had the same direction with the runner hand of rotation. The frequency of the vortex rope in the draft tube was 0.22Hz. The low frequency pressure fluctuation frequency was mainly caused by the vortex rope. In order to reduce the influence of vortex rope, the deflector was used in the draft tube. The vortex was eliminated by reducing the energy of vortex. The steady operation of the turbine has been the focus in recent years. At the same time, the low-frequency pressure-pulse in the turbine was an important factor which affected the stable operation of the turbine. The research result had reference value for the stable operation of tubular turbine.