Angiogenesis in a tumor region creates arteriovenous (AV) shunts that cause an abnormal venous blood oxygen saturation () distribution. Here, we applied optical-resolution photoacoustic microscopy to study the AV shunting in vivo. First, we built a phantom to image distribution in a vessel containing converged flows from two upstream blood vessels with different values. The phantom experiment showed that the blood from the two upstream vessels maintained a clear boundary for hundreds of seconds, which is consistent with our theoretical analysis using a diffusion model. Next, we xenotransplanted O-786 tumor cells in mouse ears and observed abnormal distribution in the downstream vein from the AV shunts in vivo. Finally, we identified the tumor location by tracing the distribution. Our study suggests that abnormal distribution induced by the AV shunts in the vessel network may be used as a new functional benchmark for early tumor detection.