Abstract: The codepositional mechanism of tertiary lake facies carbonate in the western Qaidam basin was realized in chemical codeposition process. The results show that: 1) the codeposition is mainly due to the adsorption and coating of carbonate. 2) It increases the depositional rate of carbonate and promotes the separation between carbonate layer and debris grain layer. 3) Under the action of some forces, the movement between the layers will easily produces fractures in carbonate layer, and the codeposition plays a role in protecting miropores, which makes the part of lake facie carbon rock zones in the western Qaidam basin become the effective reservoirs.