This article investigates the sliding mode control problem based on dynamic encoding-decoding over lossy networks with Round-Robin (RR) protocol. Firstly, considering non-encoded transmission, the sliding mode controller gain and the sliding mode region are acquired over in case of lossy networks under RR protocol. Then, considering encoded transmission, state-dependent online adjustment rules are introduced, and a dynamic-quantization-based encoding-decoding scheme is designed. Pre-designed sliding mode controller is proven to be feasible even with limited encoding length. However, due to inevitable encoding errors, the convergence domain and sliding mode region of the closed-loop system both increase. Finally, the effectiveness of this method is verified by a numerical example.