With the coming of 3G Age, the new mobile communication network is building in all-round way. Especially the indoor network construction is more complicated and important. To solve the problems about the network overlays and the cost of the construction, this paper presents a kind of broadband omnidirectional indoor antenna for the third generation mobile communication system. The proposed antenna is of an asymmetrical biconical structure. Based on theoretical analysis，the structure sizes of the antenna have been changed and optimized by HFSS. Its voltage standing wave ratio(VSWR) is less than 1.4 within the frequency range of 800 MHz-2 500 MHz. The gain is more than 2 dBi in low frequency and 5 dBi in high frequency. The antenna can meet the requirements of the wireless broadband communications for 2G and 3G systems, thus the cost of construction for indoor communication is reduced effectively.