The present invention relates to the well drilling field in the petroleum industry, particularly to film former used for coal-bed gas well drilling and preparation method thereof, drilling fluid and usage thereof. The preparation method of the film former includes: (1) controlling polycaprolactone diol and diisocyanate to have a first polymerization reaction in the presence of a polyurethane catalyst, to obtain a polyurethane prepolymer with two ends blocked by diisocyanate; (2) controlling the product of the first polymerization reaction to have a chain extension reaction with a polyalcohol; (3) controlling the product of the chain extension reaction and one part of hydroxyalkyl acrylate monomer to have an additive reaction, and then introducing the other part of hydroxyalkyl acrylate monomer and a styrene monomer, and emulsifying in water to obtain an emulsion; (4) mixing a radical initiator with the emulsion to have a second polymerization reaction.