It is considered that complex systems far from equilibrium are evolved in avalanche and self similar multiplicative cascade way, and have characteristics of multifractality. By using the ground electric field data obtained in the middle of Qinghai-Xizang plateau region, multifractal behavior of lightning discharges in thunderstorm is investigated with wavelet-based multifractal method. It is found that strong singularity and multfractality are involved in the strong lightning discharges, the minimum exponent αmin is less than -0.3, and the width of spectrum Δα is greater than 1.6, and the scaling behavior is consistent with the modified version of the binomial multiplicative cascade multifractal model. It is also shown that Δα increases with the enhancing of the discharge activity, and strong lightning happens in regions where Δα is high or is decreasing, the related reason is discussed.