The invention relates to a preparation process of an attenuant for a petroleum drilling fluid. In the preparation process, sulfuric acid and magnesium sulfate are used as displacers which not only ensure proper PH value, cannot pollute the working environment of operators and atmosphere environment and cannot corrode the equipment, but also cannot generate the bulk specific gravity of excessively-low specific gravity; and the price of the magnesium sulfate is 600RMB/t, about 200Kg of the magnesium sulfate is used for producing one ton of chromium lignin sulfonate with the value of 120RMB, which can reduce the cost by 400RMB as comparison with manganese sulphate for producing each ton of the chromium lignin sulfonate, so that the cost can be greatly reduced. The attenuant has perfect diluting and visbreaking functions after being added into drilling slurry at normal temperature of 25DEG C and high temperature of 150DEG C, can be compatible with other attenuants and has stable application performance. Since the attenuant only contains chromium, the high-temperature performance of brine is favorable, and the generated calcium sulfate waste residue has no chromium residue and can be widely applied without polluting the environment.