Abstract: To eliminate the effect of microannulus on cementing quality evaluation,it is necessary to investigate the effect of microannulus on sonic wave propagation.Experimental study and theoretical analysis were used to conduct this research.Adopting a simplified plate model,the microannulus was simulated by thin plastic film filled with water.According to the characteristics of the experimental model,the calculation formula that microannulus effects sonic wave was obtained by using the theory of sonic wave transmission in multi-medium plane layer.Experimental results showed that first sonic amplitude decreases with the increase of microannulus size,when microannulus size is increased to 0.1 mm,the sonic wave amplitude will be decreased by 70%.This will lead to an increase of received amplitude in logging and cause incorrect cementing explanation and evaluation.Both theoretical calculations and experimental study showed that sensitivity of sonic wave propagation in microannulus with air is higher than that filled with water,the effect of microannulus size on sonic wave propagation is very significant,its regularity could be used to evaluate the cementing quality by sonic logging.