Polyamide-imide (PAI) synthesized with trimellitic anhydride and 4, 4-diphenyl methane diisocyanate was cured at 100 ~400℃. Structure and thermal stability of the coatings were investigated by thermal dissolution testing, infrared analysis and thermal gravimetric analysis. It is found that the coating cured below 200℃ is brittle and poor stability; the coating cured at 200~300℃ shows good flexibility and good thermal stability. While the coating cured beyond 300℃ possesses poor flexibility. The coating cured at 250℃ has the most flexibility and thermal stability compared to at other temperature. The results show that the PAI coating curing mechanism is related to curing temperature. The coating curing below 200℃ depends on hydrogen bonding effects among short molecular chains, curing at 200~300℃ depends on hydrogen bonding and little crosslink among long molecular chains, while curing beyond 300℃ mainly depends on crosslink among short molecular chains.