The invention discloses a plugging agent while drilling for oil-based drilling fluids. The plugging agent while drilling comprises, by weight, 50-100 parts of hydrophobically modified starch and 200-300 parts of hydrophobically modified calcium carbonate. A preparation method of the plugging agent while drilling comprises the following steps: 1, uniformly mixing the hydrophobically modified starch with the hydrophobically modified calcium carbonate, and adding the obtained mixture into a reactor; 2, adding 100-200 parts by weight of ethanol into the reactor, using nitrogen to remove air in the reactor, and closing the reactor; 3, heating the reactor to make the temperature rise to 120-150DEG C, introducing nitrogen to protect, and reacting the hydrophobically modified starch with the hydrophobically modified calcium carbonate under the action of ethanol under a stirring speed of 200-500r/min for 60-120min; and 4, cooling a product obtained after the above reaction to room temperature, crushing the product, and sieving the product to obtain the plugging agent while drilling for oil-based drilling fluids. The plugging agent while drilling for oil-based drilling fluids has the characteristics of strong temperature resistance, protruding plugging effect while drilling, small influence on the performances of the oil-based drilling fluids, and no biological toxic degradation.