Resolving the Lorentz equation of electron motion in the undulator which has a field error, we obtain the change of transverse velocity of electrons. Through a Fourier transformation, we can find the influence of all kinds of field error in undulator on spontaneous spectrum. Choosing the BFEL parameters, we simulated the effects on FEL spontaneous spectrum due to field errors, and obtained the acceptable condition of all kinds of field errors in undulator.