The present paper aims to find out the relationship between pressure pulsations and operation mode, water head for a low head bidirectional tidal bulb turbine. 3D unsteady numerical simulations were performed and the accuracy of the method can meet the requirements of practical applications by comparing calculated results with the experimental data. The simulation results indicate that for a low head tidal bulb, runner rotational frequency (A) is the major component of the pressure pulsations both in vaneless space or rotor zone. The intensity of this dominant frequency is influenced by gravity effect or secondary flow, which depends on the specific operation mode and location. In the rotational zone from guide vanes to blades, the flow is largely affected by gravity effect not water head, which result in the amplitudes with dominant frequency increase with rotational radius in ebb and flood modes. On the contrary, in the rotational zone from blades to diffusion tube, there is obvious secondary flow. Therefore, the amplitudes with dominant frequency is ruled by head change instead of gravity effect in ebb and flood modes. (C) 2015 Elsevier Ltd. All rights reserved.