Abstract: The proton beam accelerated by target normal sheath acceleration (TNSA) mechanism has a wide energy spectrum, which limits its applications. In order to produce quasi-monoenergetic proton beams, the energy spectrum optimization method based on RF cavity has been investigated. At given RF voltage and frequency, the relationship of the cavity length and the optimization energy was calculated and spectrum optimization with different cavity lengths and cavity numbers was designed for different energy. For given number of cavities, only a certain energy range can get the minimum energy divergence. The energy acceptance has been analyzed. To get the final energy divergence of 2%, the energy divergence of the proton beam before entering RF cavity should not be over 15%. The energy acceptance affected by the RF frequency has also been analyzed. Finally a spectrum calculated by the particle-in-cell simulation with the divergence about 15% was optimized through the RF cavity, and a quasi-monoenergetic proton beam with the energy divergence lower than 2% was obtained.