A light source module of a projector includes a light source having a lamp axis, a fan for providing a cooling airflow, and a flow guide device disposed between the light source and the fan and for guiding the cooling airflow. The flow guide device includes a first air channel connected between a side of the light source and an air outlet of the fan, a second air channel connected between another side of the light source and the air outlet of the fan, and a first airflow control assembly disposed in the first air channel and for controlling the first air channel either in communication or non-communication states via gravity. The first airflow control assembly has a first control rotating shaft inclined relative to the lamp axis and a first included angle between the first control rotating shaft and the lamp axis is less than 90 degrees.