RF effect and space charge effect are the main sources of the emittance growth in the RF elec-tron gun.The single particle dynamics near the cathode is discussed,and the emittance growth due to RFnonlinear force and time-dependent force are studied in detail.Comparison with the emittance at the gunexit shows that the emittanca growth due to nonlinear RF effect near the cathode can not be neglected. RF effect and space charge effect are the main sources of the emittance growth in the RF elec-tron gun. The single particle dynamics near the cathode is discussed, and the emittance growth due to to RFnonlinear force and time-dependent force are studied in detail. Comparison with the emittance at the gunexit shows that the emittanca growth due to nonlinear RF effect near the cathode can not be neglected.