The invention discloses a double-modified starch fluid loss additive used for oil and gas field drilling, comprising silicified carboxymethyl starch or silicified carboxymethyl starch and carboxymethylated sodium alginate. The preparation method is that: the starch, inorganic silicate and alcohol-based dispersant are added to a reaction container, the starch and the inorganic silicate are evenly dispersed in the alcohol-based dispersant after being stirred; the alkali liquor is prepared with alkali, then, the alkali liquor with volume fraction of 40 percent is dripped into the reaction container; after the dripping of the alkali liquor, and a first-stage reaction is conducted under normal pressure, 30-50 degrees C. for at least 50 minutes; after the first-stage reaction is completed, the chloroacetic acid solution or the sodium alginate and the chloroacetic acid solution are added to the reaction container, and then the alkali liquor with volume fraction of 60 percent is dripped into the reaction container under stirring; after the dripping of the alkali liquor, a second-stage reaction is conducted under normal pressure,40-60 degrees C. for at least 90 minutes; the solid-liquid separation of the reaction product is conducted, and the dried solid is the double-modified starch fluid loss additive.