The invention relates to a preparation method of an inert dilution agent for an oil-based drilling fluid. According to the technical scheme, the preparation method comprises the following steps that 1, low-viscosity synthetic base oil is added in a beaker with a stirrer and a thermometer, and a reaction is carried out at the 50-90 DEG C for 3-6 hours, and then heating is stopped; 2, an emulsifyingagent is added into the reactor in the step 1, and stirring is conducted 3-6 hours; 3, a filtrate reducer, a wetting agent, an alkalinity regulating agent and a calcium chloride water solution are mixed in a ratio and then added into the reactor in the step 1, and stirring is conducted for 6-8 hours; 4, the mixture in the reactor is stirred at a high speed, a reaction is carried out at the temperature of 50-90 DEG C for 3-6 hours, cooling is conducted until to the room temperature, an emulsion is formed, and the emulsion is the inert dilution agent for the oil-based drilling fluid. The preparation method has the advantages that under the condition that the inhibitive ability and the shear thinning performance of the oil-based drilling fluid are not changed, the plastic viscosity can be reduced, and the drilling fluid obtained before and after addition of the inert dilution agent cannot affect construction safety even in the construction process.