In order to study the characteristics of the pressure fluctuation of bulb tubular turbine effected by tip clearance under the cavitation flow and to reveal the various tip clearance of bulb tubular turbine influence on the unsteady flow，numerical calculation and model test research prototype on whole flow passage are used to compare with pressure fluctuation characteristics on the four clearance schemes on the basis of the SST k-ω turbulence model and moving mesh． Then the tip clearance of the flow field structure is analyzed and the numerical results and model experimental results which confirmed the method can basically reflect the internal flow of the turbine are compared． The results show that with the reduction of the cavitation coefficient，the energy of the water turbine characteristics change，and hydraulic performance degraded too． And the arrival of the critical cavitation coefficient leads to efficiency to drop significantly; affected by the occurrence of cavitation intensification，pressure fluctuation amplitude is increased in the suction side，and the nearby flange of the amplitude is greater than that in the middle part of the blade and hub． When the partial pressure drops below the saturation pressure，fluctuation is no longer violent，with the phenomenon called“clipping”appearing． With the tip clearance value and cavitation occurrences aggravating in the gap，cavitation shape evolves from a gap cavitation to an airfoil one，and then leakage vortex cavitation emerges． The rotating frequency dominates the pressure fluctuation of the nearby flange on the suction side of the blade，a larger tip clearance can reduce high frequency pressure fluctuation at the flange，leakage flow will lead to more severe cavitation． Therefore，selecting the appropriate value gap is greatly significant to the bulb turbine in stable operation，lower vibration and noise．