Transient cavitating flows of a mixed flow PAT (pump as turbine) at pump mode are investigated experimentally and numerically. Radial force on principal axis is recorded and compared between pump with symmetrical and unsymmetrical tip clearance. Numerical simulation with improved cavitation model by modifying the vapor pressure is conducted, and the simulation results agree well with the experiments. Tip clearance has great influence on pump cavitation performance. The pump energy performance will deteriorate with tip clearance increasing. In addition, in comparison with the symmetrical tip clearance, the unsymmetrical tip clearance makes the pump cavitation performance worse. As the cavitation develops, the unsymmetrical tip clearance simultaneously influences the magnitude and direction of radial force, while the symmetrical tip clearance only influences the magnitude of radial force. The dominant frequencies of radial force of symmetrical and unsymmetrical tip clearances are related to the blade number and guide vane number, respectively. The maximum amplitude of force fluctuation for unsymmetrical tip clearance is 7 times that for symmetrical tip clearance. (C) 2018 Elsevier Ltd. All rights reserved.