A 8mm wave multimode conical feedhorn with a step of radius has been developed. Its mode ratios are calculated and the formulas of its radiation patterns in two principle planes-H plane and E plane, the voltage standing wave ratio (VSWE)，and the properties of beamwidth and sidelobe are given. Theoretical analysis and measurements show that in the range of frequency of 34.lGHz to 35.5GHz，its radiation patterns have been equalized in amplitude within 13dB and no sidelobe exists within 23dB.