An improved design of Faraday optical isolator at 780 nm was developed. Numerical simulation was used to confirm the design. It has advantage of more uniform magnetic field, smaller size and better isolation. This design of magnet called style “π” divides a whole hollow cylinder into 3 parts including several fan-shaped magnets. It can increase the use ratio of the remanence of magnet. The design of one-stage isolator has a volume of 52 cm3 with a volume of magnet of 18 cm3, which is small. The optical clear aperture is 5 mm with a 34.6 dB isolation and 90.9% transmission rate. Style “II” may also be optional for having a smaller volume but less uniformed magnets. The isolator developed according to style “π” has better performance than common used optical isolators in terms of atomic, molecular and optics (AMO) physics at the special wavelength of 780 nm in order to meet the demands of the ultracold atom system.