The invention relates to a transmission imaging device and a detection method thereof. The transmission imaging device comprises a light-emitting component, a first optical path conversion component,a second optical path conversion component, an image receiving device, and a supporting mechanism which are sequentially disposed from top to bottom, a detection channel is arranged between the firstoptical path conversion component and the second optical path conversion component; a to-be-detected light-transmitting product passes through the detection channel, and the light-emitting component,the first optical path conversion component, the second optical path conversion component, and the image receiving device are all fixedly supported by the supporting mechanism. By using the transmission imaging device and the detection method thereof, the deficiencies of the original optical detection structure are improved, the fine definition of the image is effectively improved, the method candetect dust and fine scratches in the light-transmitting product, enhances the directionality of the detected target light, increases the sharpness of the defect projection, has a simple structure, and is easy to manufacture and install.