Abstract: This paper discusses the electrom bean movement in the stand-wave cavity by the addition of a spacial third-harmonic component to the field.The analytic electric field distribution,which we give out,is well agree with real electric field.The paper also discusses the contribution of the third-haimonic component to the energy spread and emittance and uses the result of analysis to select the optimal electric field in high-brightness injector.By the analytic result, we give out a conclusion that RF effect will tends to zero when electron moving speed tends to light velocity.