Objective To study the effect of typical middle ear diseases on sound compensation of round window (RW) stimulation, so as to provide references for the optimal design of middle ear implants by RW stimulation. Methods The finite element model of the middle ear and cochlea was built by CT scanning and reverse engineering technique, and its reliability was also verified. On the basis of the model and by changing material properties of corresponding tissues, three typical middle ear diseases were simulated: hardening of stapedial annular ligaments, abnormal stapedial bone growth and hardening of anterior mallear ligaments. Then the response from displacement of basilar membrane (BM) was compared to analyze the impact of 3 types of middle ear diseases on sound compensation of RW stimulation. Results The stapes with abnormal bone growth severely deteriorated the equivalent sound pressure (ESP) of RW stimulation at higher frequencies, while the hardening of stapedial annular ligaments and the hardening of anterior malleus ligaments prominently decreased ESP of RW stimulation at lower frequencies. Among the 3 types of middle ear diseases, hardening of stapedial annular ligaments affected the sound compensation of RW stimulation more significantly with the amount of the ESP reduction up to 17 dB. Conclusions Middle ear diseases can deteriorate the sound compensation of RW stimulation seriously with large deterioration, Therefore, the output stimulation of the actuator should be targeted to improve the design of the middle ear implants by RW stimulation.