Compound of several optical waveguide microring resonators can extend the free spectral range and improve the filtering properties. For series coupled two-microring resonators, the perfect coupling condition is derived when the coupling coefficients between the two rings and the straight waveguides are not limited to be equal. When the light is input through the straight waveguide coupled with the smaller ring under perfect coupling condition, the changing of main resonance peak bandwidth, shape factor, and peak transmittance of the spurious modes with the coupling coefficients are calculated. The influence patterns of the coupling coefficients on these filtering characteristics are demonstrated. Calculation and analysis show that when the coupling coefficients between the two rings and the straight waveguides are different, balanced filtering characteristics can be realized, and more flexibility of design can be provided.