Millimeter wave communication is capable of supporting multi-gigabit wireless access thanks to its abundant spectrum resource. Nevertheless, the severe path loss makes it vulnerable to blockage events especially in dense urban environments. To address this issue, Intelligent Reflecting Surface(IRS) as a new technology has been introduced to provide effective reflected paths to enhance coverage of millimeter wave signals. This paper mainly discusses the millimeter wave systems assisted by IRS, deduces the average accessible rate of IRS, and compares the performance of this technology with the traditional Amplified and Forward(AF) relaying system. The analysis shows that when the number of reflecting elements reaches a certain value, the average accessible rate of IRS system is higher than that of AF relaying system. The optimal value of the reflecting elements is obtained to maximize the energy efficiency of the IRS system. The simulation verifies the correctness of the derived formula and shows that the energy efficiency of the system increases with the increase of the reachable rate when the number of reflecting elements is at the optimal value.