The invention discloses an intelligent package box thin letter detection method, and relates to the technical field that the process of detecting a letter placed in a package box is completed through an infrared emission optical coupler set, an infrared receiving optical coupler set, a partition plate with protrusive bumps, a buzzer, a signal alarm lamp, a control chip and the like. According to the realization approach, when the letter is placed on the partition plate with the protrusive bumps, one oblique corner of the letter is put up, and the contact area between the letter and the infrared receiving optical coupler set is increased, so that the process of putting in the letter and taking out the letter can be detected by the infrared receiving optical coupler set more easily. The intelligent package box thin letter detection method is suitable for detecting common large-size letters and particularly suitable for detecting thin letters. The detection method is a working principle and working method of a first intelligent package box thin letter detection mechanism.