Abstract: The flapping pectoral fins as actuators for bionic unmanned underwater vehicles (UUVs) achieve advantage in stealthiness, weight and adaptability, which have becoming as an important research area in UUVs. While oscillating in the water, the pectoral fin interacts with fluid and shows passive deformation on its span-wise direction, which is considered as an indicator for the generated thrust. So as to investigate the impact of span-wise stiffness of the flapping pectoral fins on its generated thrust, the scheme to fabricate a flexible pectoral fin with uniform span-wise stiffness is introduced. And then, a series of experiments to investigate the generated thrust and lateral force of the fabricated pectoral fins with different span-wise stiffness is carried out. Combined with the collected shots by a high speed camera, the impact of span-wise stiffness on the generated force is analyzed. The result shows that the performance of the flapping pectoral fin can be optimized by appropriate span-wise stiffness, which is also further verified by the swimming test of a robot prototype (XJmanta). Our research can be used to guide the design of flapping flexible bionic pectoral fin and optimizes the hydrodynamic performance of the ray-like underwater robots.