In this paper, we propose a novel algorithm to design an optimal non-uniform scalar quantizer for distributed video coding, which aims at achieving a coding rate close to joint conditional entropy of the quantized video frames given the side information. Wyner-Ziv theory on source coding is employed as the basic coding principle and the asymmetric scenario is considered. In this algorithm, a probability distribution model, which considers the influence of the joint distribution of input source and side information to the coding performance, is established and used as the optimality condition firstly. Then, a modified Lloyd Max algorithm is used to design the scalar quantizer to give an optimal quantization for input source before coding. Experimental results show that compared to uniform scalar quantization, proposed algorithm can improve coding performance largely, especially at low bit rate.