The invention relates to a hydrocarbon reservoir protective agent and a preparing method thereof.The hydrocarbon reservoir protective agent contains guar gum, organosilicate, sulfomethal phenolaldehy resin, white oil, calcium salt, alkylphenol ethoxylates, sorbitan fatty acid ester, hydrolysis polyacrylonitrile amine salt and water.The preparing method comprises the steps of 1, heating and evenly mixing water, guar gum, white oil, alkylphenol ethoxylates and sorbitan fatty acid ester to obtain a first mixture; 2, adding sulfomethal phenolaldehy resin, calcium salt, methyl silicate and hydrolysis polyacrylonitrile amine salt to the first mixture and conducting stirring to obtain the hydrocarbon reservoir protective agent.The hydrocarbon reservoir protective agent has high shale hydration expansion inhibiting capacity and dispersing capacity, does not affect the rheological property of drilling fluid, can effectively prevent or delay entering of drilling filtrate into the stratum and prevent or delay pressure increase of formation pores, and prolongs the hole-wall collapsing period.