The effect of low-frequency alternating-current (AC) electric fields on a transient flame, part of the field of electric field assisted combustion which has gotten great attention in the past several decades, needs to be investigated for more. The paper has made an experimental study of lean (excess air ratio 1.2/1.4/1.6) premixed CH4/N2/O2 flames effects under a 40 Hz low-frequency AC electric fields in a constant volume combustion bomb. The propagation characteristics and the combustion characteristics have been discussed and several conclusions are obtained by the experimental results. Firstly, the electric field has a significant effect on the flame shape; it can be observed that the flame is significantly stretched in the electric field direction under the low-frequency AC electric field. Secondly, the applied voltage of the low-frequency AC electric field has an impact on the flame propagation of lean combustion; the flame radius increases more quickly and the mean flame propagation speed also increases linearly with the applied voltage; the enhancement of the flame propagation is especially obvious in a larger excess air ratio. Thirdly, the low-frequency AC electric field can affect the combustion characteristic of lean combustion; the combustion peak pressure increases linearly with the applied voltage, while the peak time decreases with the applied voltage; both the initial duration and the main duration are shortened significantly with the electric fields, and they are also declined with the applied voltage. In a word, the low-frequency AC electric fields of 40 Hz can enhance the lean combustion effectively, which has a positive meaning in the further study of the electric field assisted combustion theory.