The transfer function was presented first and then other related formula was derived.The influence of the into /out of ring coupling coefficients on the bandwidth and extinction ratio of the spectral band was investigated and the design restriction among them and free spectral range was discussed.The analytical results show the bandwidth ratio of the spectral passband is almost independent of any structure parameters of the single ring microresonator when the into /out of ring coupling coefficients are very small.The calculated values of bandwidth ratios indicate that the bandwidth ratio is the key issue which limits the usage of the single ring microresonator as a practical optical filter.The influence of optical loss is also analyzed.Low optical loss will cause sharp attenuation of the filtering response and severe optical loss also can widen the bandwidth and increase the bandwidth ratio.