The transfer matrix of the series-coupled microring resonators is derived in detail based on the coupled mode theory of waveguides. The influence of the coupling coefficients, the number of the rings and loss on the transmission spectrum of the resonators are analyzed. Numerical simulation indicates that the bandwidth of the pass band will increase with the coupling coefficient, and at the same time, the number of the resonant wavelength in the pass band will also increase with the number of the rings. The microring resonator can act as filters or wavelength division multiplexer (WDM) in appropriate condition. Additionally, we find that the structure has the performance of optical delay when we launch a 50 ps (full-width at half maximum, FWHM) long Gaussian-shaped pulse into the structure. The larger the number of the rings, the more group delay we can achieve when the coupling coefficients is rather small, however, no delay effect can be observed when the coupling coefficient between the rings is strong.