The motion patterns of fin ray of a bionic undulating fin are normally divided into two types:constant amplitude and variable amplitude. The initial experimental tests reveal that there are significant differences on propulsion performences between these two motion patterns. In order to reveal the inherent cause for such differences, a bionic undulating fin was developed and the relevant kinematic equations were established. By taking the advantages of computational fluid dynamic (CFD), the pressure distribution on fin surface under different kinematic parameters was compared. The variation of non-dimensional drag coefficient with time under the two patterns was presented. Meanwhile, the variation of time-averaged non-dimensional drag coefficient with undulating frequency, amplitude and wavelength was also presented. In addition, the velocity contour and pressure contour in transversal surface along fin ray were illustrated. The results showed that:the time-averaged propulsion force increased with the increasement of frequency, amplitude and wavelength. However, the force produced by constant amplitude was always less than that of variable amplitude. Moreover, the velocity contour and pressure contour in transversal surface along fin ray between the two motion patterns were of great differences, but little differences were occurred in the vortex distribution in the longitudinal cross-section along fin, which indicated that the main influence of the two motion patterns might be from the transversal section along fin ray. A further discussion is implemented to illustrate the principle of aquatic organisms with undulating fins that propel themselves effectively. It provides a reference for the development of a high performance bionic undulating fin propulsion system.