The uniformity of gas velocity distribution in the gas inlet hood is important to guarantee the high de-dust efficiency in electrostatic precipitators. Closed to streamline numerical simulation method was developed to geometrically 1:1 simulate industrial large-scale electrostatic precipitator gas inlet hood and its complicated internal structures such as de-dust angle iron and two perforated gas distribution plates with up to ten thousands holes. The realizable k-e model was employed for the gas flow simulation. The results illustrate that the gas velocity decrease gradually through the gas inlet hood with non-uniform gas velocity distribution at the outlet which is low at the center area and high at the surrounding areas. The non-uniform gas velocity distribution in the gas inlet hood was analyzed in detailed to reveal the main influence factors on gas velocity distribution and put forward corresponding modification methods, which provided guidance for the structure retrofit of the gas inlet hood. After the retrofit, the field measurements of the gas velocities at the outlet of the gas inlet hood showed that the non-uniform index of gas velocity distribution decreased greatly from 0.355 to 0.244, which indicated that the uniformity of gas velocity distribution is improved to realize higher electrostatic precipitator performance with lower dust emissions. Therefore, closed to streamline numerical simulation method can reveal gas velocity distribution in the complicated structure of electrostatic precipitator gas inlet hoods and then direct its design and retrofit of electrostatic precipitator gas inlet hoods.