The invention belongs to the field of chemical mechanical polishing and particularly relates to a control method for controlling the surface roughness of the cobalt barrier layer of a multi-layer copper wiring. According to the method, firstly, an alkaline polishing solution is prepared. Secondly, the polishing treatment is conducted for 1-5 minutes under the following process conditions of flow rate: 100-500 ml/min; temperature: 15-30 DEG C; polishing head rotation speed: 57-150 rpm; polishing disk rotation speed: 63-150 rpm; and polishing pressure: 6.89-27.56 kpa. The alkaline polishing solution comprises the following components in weight percentage relative to the total weight of the alkaline polishing solution: 0.1-1% of chelating agent, 1-5% of nonionic surfactant and 1% of SiO2, wherein the final pH value of the polishing solution is 8-12. The above method overcomes the defect in the prior art that during the conventional chemical and mechanical polishing process of the copper wiring, the surface roughness of the cobalt barrier layer is mainly controlled by an acidic polishing solution containing an oxidizing agent.