Let $\mathbf{ U}$ be the quantized enveloping algebra and $\dot{ \mathbf{ U}}$ its modified form. Lusztig gives some symmetries on $\mathbf{ U}$ and $\dot{ \mathbf{ U}}$. Since the realization of $\mathbf{ U}$ by the reduced Drinfeld double of the Ringel-Hall algebra, one can apply the BGP-reflection functors to the double Ringel-Hall algebra to obtain Lusztig's symmetries on $\mathbf{ U}$ and their important properties, for instance, the braid relations. In this paper, we define a modified form $\dot{ \mathcal{ H}}$ of the Ringel-Hall algebra and realize the Lusztig's symmetries on $\dot{ \mathbf{ U}}$ by applying the BGP-reflection functors to $\dot{ \mathcal{ H}}$.
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