The characteristics of proton beams generated from ultra-intense laser-foil interactions under varied focal offsets are experimentally studied on XL-Ⅱ laser facility.The experimental results show that when strong femtosecond prepulses are presented before the main pulse, the conversion efficiency from the laser energy to the protons is enhanced by 3 orders of magnitude with effective improvement of the beam collimation by properly defocusing the laser pulses. The depression of the prepulse effect on the proton acceleration is believed to be one of the main reasons for the significant optimization. Besides, the particle-in-cell (PIC) simulation results indicate that the population of moderate energy electrons increases under large focal offsets, which in turn effectively improves the acceleration electric field and the beam quality.