The invention relates to a method for detecting the internal gap of a composite part, which comprises the following steps: pre-installing an air bearing seat, an air bearing cover and an air bearing; measuring the distance L between the air bearing seat end face and the air bearing cover end face by using a camera and a telecentric lens; calculating a gasket thickness N through the size M on the air bearing cover and the gap X of the air bearing between the air bearing cover and the air bearing seat; selecting the appropriate gasket size and quantity, and mounting the air bearing seat, the air bearing cover and the air bearing in place; and verifying whether the gasket is in place and re-installing the gasket if not. The method simplifies the measurement process and shortens the measurement time; after the installation in place, the gap is confirmed, the accuracy of the gap after the installation in place is ensured, the measurement accuracy is within 0.01m, and air inlet components of the same batch have good consistency.