The evolution of spikes formed on silicon surface by irradiating femtosecond laser pulses in SF6 as the increase of incident pulse energy is experimentally investigated. The spike height increases with the increase of pulse energy at first and then decreases with the pulse energy continuously increasing. The increase of spike height is all due to the material ejection at the initial stage. While the high energy can not penetrate into the deep layer of silicon completely during the initial several hundreds of laser pulses, more and more energy accumulates on the topmost layer and the silicon surface is maintained in the molten state, which hinders the formation of spike structure. This leads the decrease of the effective number of pulses interacting with silicon, together with the decrease of the spike height.