The invention relates to the field of oil and gas recovery, in particular to a polyamide copolymer suitable for use as a low-temperature viscosity reducing agent, a preparation method thereof and an oil-based drilling fluid. The polyamide copolymer contains structural units provided by long chain binary acid, a structural unit provided by a diamine compound and an end group provided by a diol compound, wherein the long chain binary acid is one or more of compounds represented by formula (1) and dimeric fatty acids, the diamine compound is a compound represented by formula (2), and the diol compound is a compound represented by formula (3); formula (1): HOOC-R1-COOH; formula (2): H2N-R2-NH2; formula (3): HO-R3-OH. The polyamide copolymer provided by the invention can realize the constant rheological property of the oil-based/synthetic-based drilling fluid by reducing the low-temperature viscosity shear force, does not need to strictly screen the core treatment agent, and has better universality.