Abstract: Micro-ring resonator has been attracting attentions in studies of filters, modulators, lasers, sensors and other fields owing to the characteristics of highly integrated, low power consumption and high sensitivity. The transmission characteristics of micro-ring resonator directly determine the performance of micro-optical device for its key component of various integrated optical devices. The causes of and influences on resonant spectrum of ripple noise were analyzed by the combination of micro-ring resonator theory and Fabry-Perot Interferometer (FPI). Coupling experiments were designed to obtain and analyze the free spectrum range (FSR) and amplitude of ripple noise. The experimental results show that coupling gratings are the cause of ripple noise, which is well consistent with theoretical analysis. The relationship between etching depth of coupling gratings and ripple amplitude was found and analyzed. The study provides a reference for further optimization of micro-ring resonator and error analysis of micro-ring resonator system.