This paper presents a bidding model based on a two-stage electricity market structure to solve the bidding difficulties which emerging retail companies would face under the Spread-Rebate mechanism. In order to increase the market efficiency while reduce the risk of retailers, a general retailer agent, surrogate for all retailers, is modeled as the only buyer in the market who distributes the spread tariff among retailers after market clearing. In the upper stage, a bilateral Bayesian game model with asymmetric information between the generation companies and the general retailer agent is structured, whereas in the lower stage, a decision-making model with demand side elasticity for retail companies is built. Finally, an illustrative simple example is given to demonstrate the viability and effectiveness of the proposed method. This work is supported by the Chunhui Program of Ministry of Education (No. Z2016145), National Natural Science Foundation of China (No. 51677020 and No. 51877181), and Educational Commission of Sichuan Province of China (No. 18CZ0018).