The invention relates to an isolating liquid used in oil well cementing, comprising fresh water, suspension agent, thinner agent and aggravation agent. The invention is characterized in that: the suspension agent is made from hydroxy ethyl cellulose, carboxymethyl cellulose, xanthan and na-bentonite; the thinner agent is made from sodium lignosulphonate, sodium dodecyl benzene sulfonate and alkyl phenol polyoxyethylene ether; the aggravation agent is made from barite, fly ash, limestone and slag. When preparing the isolating liquid, the suspension agent is dissolved in water for one hour, thereafter thinner agent is added. Before cementing, the aggravation agent is added into the solution and mixed evenly. The isolating liquid is characterized by low viscosity, good rheological performance, slow critical velocity and being easy to form turbulent flow, so as to effectively substitute and wash mud, prevent forming mixed slurry of cement slurry and mud. Meanwhile, the invention can be compatible with ordinary cement slurry and mud system, thus diluting and improving cement slurry rheological performance.