Abstract: Two microwave resonant cavity with different structures for preparing optical diamond film materials were designed. The Hill shape reentrant cavity can provide enough space for microwave to resonate inside, helping to excite high-density plasma. The improved structure with tilted up substrate holde can obtain high quality film with less impurity. In the optimization process of the second design, it was found that the transition structure with cone can obtain stronger electric field intensity and higher deposition rate than the direct connection style. Gas supply ways and flow rate were optimized, two kinds of gas supply modes were proposed. Mode I was that gas enter from one center hole on the top of the cavity, and mode Ⅱ was a circular path on the top. The results show that the air inlet way with mode I is favorable for the deposition of uniform film, and the optimal gas flow rate range is 5-10 m/s. The designed microwave cavity can be applied to the preparation of high quality optical diamond film.