The invention discloses a polyamide imide, a film and a preparation method thereof. Diamine containing amide structure and other diamines and various dianhydrides are copolymerized; the high-performance polyamide imide and the film thereof are prepared by copolycondensation. The polyamide imide of the invention has large molecular weight, excellent mechanical property, high thermostability, excellent firm forming ability and low thermal expansion coefficient and has favourable adhesive property with the substrate. The invention can be used for preparing single-layer or multi-layer free-adhesive flexible copper clad laminates.