Photoacoustic imaging, which can provide the maximum intensity contrast in tissue depth imaging without ionizing radiation, will be a promising imaging trend for tumor detection. In this paper, a column diffusion fiber was employed to carry a pulsed laser for irradiating stomach directly through esophagus based on the characteristics of gastric tissue structure. A long focused ultrasonic transducer was placed outside the body to detect photoacoustic signals of gastric tissue. Phantom and in vitro experiments of submucosal gastric tumors were carried out to check the sensitivity of scanning photoacoustic tomography system, including the lateral and longitudinal resolution of the system, sensitivity of different absorption coe±cient in imaging, capability of transversal detection, and probability of longitudinal detection. The results demonstrate that our innovative technique can improve the parameters of imaging. The lateral resolution reaches 2.09 mm. Then a depth of 5.5mm with a longitudinal accuracy of 0.36mm below gastric mucosa of early gastric cancer (EGC) has been achieved. In addition, the optimal absorption coe±cient differences among absorbers of system are 3.3–3.9 times. Results indicate that our photoacoustic imaging (PAI) system, is based on a long focusing transducer, can provide a potential application for detecting submucosal EGC without obvious symptoms.