Patent infringement lawyer's letter is a letter sent by a professional lawyer to the infringer after the obligee discovers the infringement, asking him to stop the infringement. In the case of high litigation cost, lawyer's letter is not only becoming a fast and convenient non-litigation weapon for obligees to stop infringement and safeguard their own rights and interests, but also plays a role in proving that sellers know that the products they sell are subjectively and intentionally infringing patent rights in subsequent litigation. Properly used, it will play the role of "defeating the enemy without fighting", but improperly used, it will "startle the snake" and even constitute unfair competition.