Photoacoustic imaging is an emerging medical imaging technique combining the high contrast of optical imaging and the superior penetration depth of ultrasound imaging. It can provide detailed three-dimensional images of the feeding vessels of tumors, which is essential for the early diagnosis of cancer. In this study, we developed a transmissive coaxial photoacoustic digestive tract endoscopic imaging system using internal optical illumination and external ultrasonic detection. The phantom imaging results indicated that the transmissive system has a much better performance than the reflective non-coaxial system with a 43.3 dB higher Signal-to-Noise Ratio (SNR) and 28.4% better imaging depth. Ex vivo imaging results indicated that the transmissive system has a 9.7 dB higher SNR at a depth of 10.7 mm compared with the reflective system. The transmissive coaxial photoacoustic endoscopic imaging system, which exhibited a higher SNR and better imaging depth than the reflective system, shows that this design improves the detection sensitivity effectively. The significant improvements suggest that the developed photoacoustic endoscopy has great potential for translation into a broad range of clinical applications in gastroenterology.