A drilling fluid additive composition comprising a viscosity reducer, an emulsifier, and a rheological modifier, wherein: the viscosity reducer is a double-block copolymer mainly consisting of a block A and a block B, wherein, the block A is composed of structural units represented by formula, the block B is composed of structural units represented by formula, and the number-average molecular weight of the block A is 1,500-10,000, wherein, R 1, R 1', R 4, R 4', and R 6 are independently selected from H and C1-C6 alkyl; R 2 and R 2' are independently selected from H, C1-C6 alkyl, -O-C -R 3 and -COOR 3, and at least one of R 2 and R 2' is selected from -O-C -R 3 or -COOR 3, and R 3 is selected from C1-C6 alkyl; R 5 is selected from C1-C8 alkylene; X is selected from -NH- or -O-; the emulsifier is one or more of compounds represented by formula; in the formula, each of the two R 1 groups is independently selected from C14-C30 alkyl optionally substituted by group Y and C14-C30 unsaturated alkyl with carbon-carbon double bonds optionally substituted by group Y, and the group Y is independently selected from the groups represented by the following formulae: n is an integer in a range of 1-8; n X'-es are independently selected from H and -C -R 2, and at least one X' is -C -R 2, R 2 is selected from carboxyl, C1-C6 alkyl, C3-C8 cycloalkyl, C1-C6 alkyl substituted by carboxyl, C3-C8 cycloalkyl substituted by carboxyl, C3-C8 cycloalkyl substituted by carboxyl and C1-C4 alkyl, C2-C6 unsaturated alkyl with a carbon-carbon double bond, C3-C8 unsaturated cycloalkyl with a carbon-carbon double bond, C2-C6 unsaturated alkyl with a carbon-carbon double bond substituted by carboxyl, and C3-C8 unsaturated cycloalkyl with a carbon-carbon double bond substituted by carboxyl and C1-C4 alkyl; the rheological modifier is a dimer acid-organic amine copolymer comprising structural units from dimer acid, structural units from alkylamine and structural units from arylamine, of which the dimer acid is a dimer of oleic acid and linoleic acid, the alkylamine is one or more of C10-C20 alkyl primary amines, and the arylamine is one or more of aniline and aniline substituted by C1-C3 alkyl at one or more sites on benzene ring. the viscosity reducer is a double-block copolymer mainly consisting of a block A and a block B, wherein, the block A is composed of structural units represented by formula, the block B is composed of structural units represented by formula, and the number-average molecular weight of the block A is 1,500-10,000, wherein, R 1, R 1', R 4, R 4', and R 6 are independently selected from H and C1-C6 alkyl; R 2 and R 2' are independently selected from H, C1-C6 alkyl, -O-C -R 3 and -COOR 3, and at least one of R 2 and R 2' is selected from -O-C -R 3 or -COOR 3, and R 3 is selected from C1-C6 alkyl; R 5 is selected from C1-C8 alkylene; X is selected from -NH- or -O-; the emulsifier is one or more of compounds represented by formula; in the formula, each of the two R 1 groups is independently selected from C14-C30 alkyl optionally substituted by group Y and C14-C30 unsaturated alkyl with carbon-carbon double bonds optionally substituted by group Y, and the group Y is independently selected from the groups represented by the following formulae: n is an integer in a range of 1-8; n X'-es are independently selected from H and -C -R 2, and at least one X' is -C -R 2, R 2 is selected from carboxyl, C1-C6 alkyl, C3-C8 cycloalkyl, C1-C6 alkyl substituted by carboxyl, C3-C8 cycloalkyl substituted by carboxyl, C3-C8 cycloalkyl substituted by carboxyl and C1-C4 alkyl, C2-C6 unsaturated alkyl with a carbon-carbon double bond, C3-C8 unsaturated cycloalkyl with a carbon-carbon double bond, C2-C6 unsaturated alkyl with a carbon-carbon double bond substituted by carboxyl, and C3-C8 unsaturated cycloalkyl with a carbon-carbon double bond substituted by carboxyl and C1-C4 alkyl; n is an integer in a range of 1-8; n X'-es are independently selected from H and -C -R 2, and at least one X' is -C -R 2, R 2 is selected from carboxyl, C1-C6 alkyl, C3-C8 cycloalkyl, C1-C6 alkyl substituted by carboxyl, C3-C8 cycloalkyl substituted by carboxyl, C3-C8 cycloalkyl substituted by carboxyl and C1-C4 alkyl, C2-C6 unsaturated alkyl with a carbon-carbon double bond, C3-C8 unsaturated cycloalkyl with a carbon-carbon double bond, C2-C6 unsaturated alkyl with a carbon-carbon double bond substituted by carboxyl, and C3-C8 unsaturated cycloalkyl with a carbon-carbon double bond substituted by carboxyl and C1-C4 alkyl; the rheological modifier is a dimer acid-organic amine copolymer comprising structural units from dimer acid, structural units from alkylamine and structural units from arylamine, of which the dimer acid is a dimer of oleic acid and linoleic acid, the alkylamine is one or more of C10-C20 alkyl primary amines, and the arylamine is one or more of aniline and aniline substituted by C1-C3 alkyl at one or more sites on benzene ring.